Backlight having discharge tube, reflector and heat conduction member contacting discharge tube

ABSTRACT

A display device includes a backlight having a discharge tube and a reflector. A heat conduction member is attached to the reflector in contact with the discharge tube, so that a part of the discharge tube is locally cooled by the heat conduction member. Liquid mercury is collected at a first position in the discharge tube, and the backlight is assembled so that the heat conduction member or other cooling device is located at the first position. Also, the display device includes an optical sheet having a diffusion portion having projections containing scattering material particles.

This Application is a Divisional of parent application Ser. No.10/005,259, filed Dec. 4, 2001 now U.S. Pat. No. 7,164,224.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a backlight, of a display device, having adischarge tube emitting light during discharge in a lean gas and to amethod of manufacturing a backlight, and a display device.

2. Description of the Related Art

A backlight, of a display device such as a liquid crystal displaydevice, uses a light source device comprising one or a plurality ofdischarge tubes and a reflector. The discharge tube is a cold cathodetube, in which mercury and argon (Ar) gas or neon (Ne) gas are sealedand a fluorescent material is coated on the tube wall. Mercury gasgenerates ultraviolet light during discharge, and the ultraviolet lightimpinges against the fluorescent material and generates visible light.

The backlight of most of liquid crystal display devices includes a lightguide plate. In one example, two light source devices are disposed onopposite sides of the light guide plate in such a manner as to face eachother. Each light source device comprises two discharge tubes and areflector. In this arrangement, two discharge tubes having a diameter ofseveral millimeters are disposed in a narrow region of not greater than10 mm. Therefore, the ambient temperature around the discharge tubesoften reaches 70° C. or more.

The light intensity-temperature characteristics of a discharge tube havea tendency such that the light intensity drops in a high temperatureregion for the following reason. First, considering the amount ofultraviolet light generated by the mercury gas, the amount issubstantially proportional to the mercury gas concentration and thecurrent. On the other hand, mercury gas has the property of absorbingultraviolet light, and the absorption factor changes exponentially, withthe product of the mercury gas concentration, over the distance theultraviolet light must travel (the transmission factor changes as theconcentration and the distance of transmission become greater). Theultraviolet light is converted into visible light by the fluorescentmaterial coated on the tube wall. The product of the diameter of thedischarge tube and the concentration of the mercury gas determines theprobability of the incidence of one UV photon on the fluorescentmaterial. From the explanation given above, the light intensity I of thevisible light can be expressed as follows, wherein “d” is the tubediameter, “n” is the mercury gas concentration (a function of thetemperature of discharge tube) and “J” is the current.I˜k(J×n)×exp(−b×n×d)  (1)(where k and b are proportional constants.)

Equation (1) shows that I is likely to assume the maximum value for apredetermined mercury gas concentration n, and when the mercury gasconcentration becomes higher than this predetermined concentration n,the intensity of the visible light drops. The mercury gas concentrationchanges exponentially with the temperature of the mercury gas and,therefore, the brightness becomes lower in a high temperature region asthe tube temperature becomes higher. Since the tube temperature becomeshigher as the current increases, the intensity of the visible raydecreases when the current is increased at a predetermined ambienttemperature. These decreases cause a problem when the brightness of thebacklight is to be increased.

Japanese Unexamined Patent Publication (Kokai) No. 5-225819 disclosescontrol of the brightness of a discharge tube by fitting a cooling metaldevice to the discharge tube to cool the whole discharge tube.

Japanese unexamined Patent Publication (Kokai) No. 60-16813 discloses alight source for a copying machine. This light source comprises afluorescent lamp, a lamp heater encompassing the fluorescent lamp and aheat pump disposed at a notched portion of the lamp heater. The heatpump absorbs heat from the tube wall of the fluorescent lamp andcontrols the light intensity of the fluorescent lamp by controlling themercury vapor pressure in the fluorescent lamp tube.

However, it is difficult to apply the technology of fitting the coolingmetal device for the discharge tube to the light source device of thebacklight of the display device. That is, as the tube diameter is smallin the backlight of the display device and a reflector exists around thedischarge tube, a large cooling metal device cannot be fitted to thedischarge tube. Also, leakage of current through the cooling metaldevice becomes large and, as the backlight of the display device hassmaller power consumption than the fluorescent lamp for ordinaryillumination, the discharge tube is likely to be excessively cooled. Forthese reasons, this technology is not practical.

The light source device having a lamp heater that encompasses thefluorescent lamp cannot be used for the backlight of the liquid crystaldisplay device.

Japanese Unexamined Patent Publication (Kokai) No. 2000-323099 disclosesa method of fabricating a fluorescent lamp in which liquid mercurygathers at the central portion of a fluorescent lamp to prevent thecentral portion of the fluorescent lamp from becoming dark for severalseconds of its use after the lamp is left standing for a long time in acold environment, because liquid mercy gathers at the ends of thefluorescent lamp. This prior art discloses that when the fluorescentlamp is cooled and the temperature of its central portion becomes lower,by about 10° C., than the temperature at the ends thereof, liquidmercury gathers at the central portion of the fluorescent lamp. Inpractice, however, almost all the liquid mercury in the fluorescent lampdoes not gather at one position in the fluorescent lamp.

A study of small diameter discharge tubes, for use in the backlight,conducted by the inventors has revealed that almost all the liquidmercury does not gather at a position remote from the end portions of adischarge tube having the inner diameter of 5 mm or below even when thedischarge tube is assembled into a backlight. When electric current isapplied to activate the discharge tube, liquid mercury generally gathersat one of the end portions because of the asymmetry of the waveform.

Even when the waveform is symmetric, liquid mercury that is arbitrarilydistributed in the discharge tube does not gather easily at one positionin the discharge tube of a backlight using a thin glass tube of 5 mm orless, because the tube is thin and contains an amount of liquid mercuryconsiderably greater than the amount of the gaseous mercury necessaryfor discharge. According to experiments conducted by the inventors, atime of 200 to 1,000 hours is necessary to collect the liquid mercury atone position in the discharge tube. During the process in which liquidmercury gathers at one position, degradation of the fluorescent lampproceeds and the brightness drops.

According to one aspect, the present invention provides a backlightcapable of improving brightness by forming a most-cooled portion at apredetermined position in a discharge tube. However, it has been foundthat, in such a backlight, the desired improvement of brightness cannotbe achieved unless liquid mercury is collected at the predeterminedposition. In another aspect, therefore, the present invention provides abacklight in which liquid mercury is collected at a predeterminedposition of a discharge tube, and this predetermined position is themost cooled portion.

The display device having the backlight containing the discharge tubeinvolves another problem in that, even when the current supplied to thedischarge tube is increased, the brightness does not increase much.

In the case of the liquid crystal display device using linearlypolarized light, only a half of light of a non-polarized light source isutilized, hence, the utilization efficiency of light is low. A proposalis therefore made to dispose a polarization separating element in thebacklight of the display device to improve utilization efficiency oflight. The polarization separating element comprises a reflection typepolarization plate (polarization separating sheet) sandwiched between alight guide plate and a liquid crystal panel. The reflection typepolarization plate allows first linearly polarized light of the ray oflight traveling from the light guide plate to the reflection typepolarization plate to transmit therethrough but reflects second linearlypolarized light having a plane of polarization crossing orthogonally theplane of polarization of first linearly polarized light. The plane ofpolarization of second linearly polarized light, that is again madeincident to the light guide plate, is converted by means for convertingthe second linearly polarized light to first linearly polarized light.Therefore, the second linearly polarized light travels again as firstlinearly polarized light from the light guide plate to the reflectiontype polarization plate and is transmitted through the latter in thisway, the utilization efficiency of the light can be improved, and adisplay device having higher brightness can be achieved.

In the conventional backlight, a diffusion reflection plate is disposedas first means for converting second linearly polarized light to firstlinearly polarized light below the light guide plate. Second linearlypolarized light that is reflected by the reflection type polarizationplate and is again made incident to the light guide plate is scatteredand reflected by the diffusion reflection plate to non-polarized light.As the non-polarized light is thus made incident to the reflection typepolarization plate, at least a part of the second linearly polarizedlight can be utilized, and utilization efficiency of light can beimproved in comparison with the case where second linearly polarizedlight is not at all utilized. However, a part of second linearlypolarized light is scattered and reflected by the diffusion reflectionplate, is scattered in the periphery of the light guide plate withouttraveling from the light guide plate to the reflection type polarizationplate, and is absorbed by the light source and the casing. Therefore,the utilization efficiency of light is still limited.

In another example, a λ/4 plate is disposed as second means forconverting second linearly polarized light into first linearly polarizedlight below the reflection type polarization plate, and an isotropicmetal mirror is disposed below the light guide plate. Second linearlypolarized light reflected by the reflection type polarization platepasses through the λ/4 plate, changes to left (right) circularlypolarized light, is reflected by the isotropic metal mirror, changes toright (left) circularly polarized light, again passes through the λ/4plate and changes to first linearly polarized light. Since firstlinearly polarized light transmits through the reflection typepolarization plate, utilization efficiency of light is improved. In thiscase, however, the isotropic metal mirror absorbs the rays of light, sothat utilization efficiency of light is limited, too.

In addition, backlights of the liquid crystal display device include a“side light type” backlights and a “direct illumination type”backlights. The side light type backlight includes a light guide plateand a light source disposed on the side of the light guide plate, andhas the advantage that a thin liquid crystal display device can beprovided. The direct illumination type backlight includes a light sourceradiating the ray of light to the liquid crystal display device, and hasthe advantage that a high brightness of the liquid crystal displaydevice can be accomplished. However, the direct illumination typebacklight cannot easily provide a liquid crystal display device that isthin and has low power consumption, and involves the problem thatnon-uniformity of brightness is likely to occur. Therefore, the sidelight type backlight has gained a wider application in recent years.

Besides the light guide plate and the light source described above, theside light type backlight includes a reflection mirror (reflection film)disposed below the light guide plate (on the far side from the liquidcrystal panel) and an optical sheet disposed above the light guide plate(on the near side to the liquid crystal panel). The light outgoing fromthe light source is made incident to the light guide plate. While thelight propagates in the light guide plate, the reflection mirrorreflects a part of the light, and the reflected light goes from thelight guide plate and is made incident to the liquid crystal panelthrough the optical sheet.

The optical sheet regulates the brightness distribution of lightoutgoing from the light guide plate. In other words, since the lightgoing from the light guide plate contains a large quantity of the lightthat describes a large angle to the normal to the light guide plate, theoptical sheet mainly converts the light describing a large angle to thenormal to the light guide plate into the light describing a small angleto the normal to the light guide plate.

FIG. 123 illustrates a prism sheet as one of the optical sheets. Theprism sheet 1 is a transparent sheet having a large number of prisms 2formed thereon. The light X made incident to the prism sheet 1 with alarge angle to the normal of the prism sheet 1 goes from the prism sheet1 with a small angle to the normal of the prism sheet 1. Therefore, anobserver of a liquid crystal panel behind the prism sheet 1 can easilyview the liquid crystal panel from the front surface. On the other hand,the prism 2 reflects the light Y that is made incident to the prismsheet 1 along the normal of the prism sheet 1. Therefore, this ray Yreturns.

Incidentally, Japanese Unexamined Patent Publications (Kokai) No.6-273762, No. 8-146207, No. 9-15404, No. 10-97199, No. 10-246805 and No.2000-56105 disclose examples of the prism sheets and the scatter sheets.Japanese Unexamined Patent Publication (Kokai) No. 11-329042 describesan example of the planar light source.

As described above, the prism sheet 1 has fine prisms 2 formed on thesheet surface in order to allow the light, that is made incident to theprism sheet 1 at an angle in an oblique direction, to enter in the frontsurface direction or at an angle approximate to the front surface. Inthis prism sheet 1, the quantity of the outgoing light within theoutgoing angle range determined by the shape of the prisms 2 is large,and the quantity of the outgoing light outside the outgoing angle rangedrops drastically. In other words, the brightness distribution of thelight made incident to the liquid crystal panel changes drastically.Therefore, the conventional technology combines the prism sheet 1 withthe scatter sheet containing scattering material particles to achieve awide brightness distribution such that the quantity of the light at anangle in the normal direction becomes maximum and becomes progressivelysmaller as the angle becomes greater, from the normal direction. Thelight Y made incident to the prism sheet 1 along the normal to the prismsheet 1 is returned to the light guide plate side, and utilizationefficiency of light drops. Further, the light X that is inclined to oneside leaves from near one of the ends of the light guide plate, and thelight Z inclined to the other side outgoes from near the other end oflight guide plate. This tendency remains even after the light istransmitted through the prism sheet.

The production cost of the prism sheet 1 is high because the fine prisms2 must be fabricated accurately in the prism sheet 1. The prism sheet 1itself does not have a light absorbing property, but the ray of lightreturned towards the light guide plate is absorbed by the reflectingmirror, the light source, the casing frame, etc, with a drop inutilization efficiency. When the prism sheet and the scatter sheetcontaining the scattering material particles are used in combination,the production cost increases due to an increase in the cost of thesheet itself and an increase in the cost and the number of assemblysteps. Further, a problem of a drop in the yield occurs because dustappears between the prism sheet and the scatter sheet.

As the thickness of the liquid crystal display device is required to besmaller and smaller, the light guide plate becomes smaller and smaller,too. When the thickness of the light guide plate is reduced, however,the quantity of incoming light from the sides of the light guide platebecomes small. Therefore, a reduction of thickness of the side edge typebacklight is limited.

To cope with a reduction in the thickness of the light guide plate, amethod has been proposed to input the rays of light from the upper orlower surface of the light guide plate (e.g. Japanese Unexamined PatentPublication (Kokai) No. 11-329042). This proposal employs a structure inwhich a part of a flexible film is curved to form a cylindrical portionand the light source is positioned in this cylindrical portion so as toguide the light received from the cylindrical portion to other portionsof the flexible film. However, according to this construction, a part ofthe light irradiated from the light source onto a part of thecylindrical portion is guided to other portions, but another part of thelight travels to the other side of the flexible film. Furthermore, thescatter-reflection layer reflects still another part of the lightirradiated from the light source to the cylindrical portion, and thelight returns to the light source lowering, thereby, the utilizationefficiency of light.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a backlight capableof improving brightness by forming the most cooled portion at apredetermined position of a discharge tube, a display device and aproduction method thereof.

It is another object of the present invention to provide a backlight ofa display device that has high utilization efficiency of light and canbe used in a liquid crystal display device, for example.

It is still another object of the present invention to provide a displaydevice capable of greatly changing brightness.

It is still another object of the present invention to provide anoptical sheet having a suitable brightness distribution and capable ofbeing economically produced, and an illumination apparatus.

It is still another object of the present invention to provide anoptical member capable of inputting large quantities of light even whenit assumes the form of a thin sheet-like optical member.

It is a further object of the present invention to provide a lightsource device having a long operating life.

A backlight, according to the present invention, includes a dischargetube, a reflector for reflecting a ray of light emitted from thedischarge tube, and a heat conduction member attached to the reflectorin contact with a part of the discharge tube, so that a part of thedischarge tube is locally cooled by the heat conduction member. In thisconstruction, there is an optimum value for a concentration of a lightemitting substance charged in the discharge tube, such as a mercury gas,to make the light emission quantity maximum. The optimum value issubstantially constant irrespective of a current. On the basis of thisresult, the inventors have devised a method for maintaining theconcentration of the mercury gas irrespective of the gas temperature.

Assuming that the optimum concentration of the mercury gas is “n” andthe inner capacity of the discharge tube is “V”, the concentration isbelow “n” irrespective of the temperature if mercury is chargedonly tonV or below in the discharge tube. At present, however, mercury in themount 1,000 times the necessary amount is charged to secure life of thedischarge tube.

Of the amount of mercury described above, when the excessive amount withrespect to nV is concentrated on a portion of the discharge tube whosetemperature can be controlled, the mercury gas pressure in the dischargetube is equal to the saturation vapor pressure of mercury at thetemperature of liquid crystal at the controlled portion of the dischargetube. Micro regions having an equal temperature are defined when thetemperature distribution in the discharge tube reaches an equilibriumunder the condition where the temperature of the portions of thedischarge tube other than the temperature controlled portion is higher.Since input/output of the mercury atoms is equal among these microregions, the mercury gas pressure is equal throughout the whole region.As to the equation of a state of the mercury gas, on the other hand, theconcentration of the mercury gas (atomic concentration) of each microregion is inversely proportional to the temperature of the mercury gasbecause the temperature follows the equation in the respective microregions. It can be therefore said that the mercury gas concentration(atomic concentration) becomes lower in a higher temperature range. Whenthe temperature inside the discharge tube is elevated to a level higherthan that of the temperature controlled portion, the concentration ofthe mercury gas reaches maximum at the temperature controlled portionand is lower at the other portions because the pressure reaches amaximum at each point and the pressure is constant at each point.

When the temperature of the portion of the discharge tube, thetemperature of which is to be controlled, is set to the temperature thatgives the optimum mercury gas concentration, the discharge tube can bebrought, as a whole, into a condition of maximum light emissionquantity. To accomplish this temperature condition, the following meanscan be employed.

A heat conduction member for releasing heat from the discharge tube isbrought into the temperature-controlled portion and is fitted to thereflector. The heat conductivity of this heat conduction member is setto the range that can be controlled to the temperature described above.When the heat conduction member is at an ordinary room temperature (20°C.), heat generated by the discharge tube must be escaped to provide atemperature difference of 50° C. in order to keep the surfacetemperature of the discharge tube that gives maximum brightness. Heatconduction in the transverse direction is small inside the dischargetube. Therefore, it is necessary to discharge only heat generated in thecontact area with the heat conduction member. Assuming that theproportion of energy converted into heat is η, from the energy inputtedto the discharge tube, and that this heat is uniformly discharged on theentire surface of the discharge tube, the calories per unit length isapproximated to η×W/L. Heat of η×a×W/L is generated in the length a inwhich the heat conduction member keeps contact. Therefore, this heatmust be escaped to provide the temperature difference of 50° C. In otherwords, the heat resistance of the heat conduction member must be atleast 0.02×η×a×W/L (W/K).

The heat conduction member is a solid article. It is preferably anonmetal. Further, the heat conduction member is made of at least one ofheat conductive resin, heat conductive rubber and heat conductiveadhesive. The upper limit of energy consumption of the discharge tube isdetermined so that heat radiation by using a silicone rubber can attainan optimum temperature. A hole smaller than the tube diameter is boredin the solid article, and the discharge tube is fixed to the hole. Theheat conduction member is bonded to the reflector or to the dischargetube in a boding state equal to or stronger than a hydrogen bond.Alternatively, the heat conduction member uses a viscous material. Theportion at which the heat conduction capacity drops most greatly is theinterface between substances. It is known that heat conductivity of theinterface is the highest on the bonded interface, next on the adheringinterface and the lowest on the interface merely keeping contact. Tolower the temperature of the discharge tube, therefore, the heatconduction member is more preferably bonded to both discharge tube andreflector. Heat conductivity is better in the structure in which theheat conduction member itself is fixed to the discharge tube (or thereflector) than the structure in which the heat conduction member andthe discharge tube (or the reflector) are bonded through an adhesivematerial disposed separately. The former is preferred to the latter.Preferably, the heat conduction member is in a stronger bonding statewith at least one of the discharge tube and the reflector than ahydrogen bond. (This tendency is particularly remarkable when a siliconetype heat conduction member is used).

The present invention employs the construction for evaporating anoperating material for heat conduction in the proximity of the dischargetube. Therefore, this construction can deprive the discharge tube ofheat and can speed up heat radiation. The present invention uses a heatsiphon for heat conduction, condenses the operation material at theupper part and refluxes it by gravity. The present invention can alsouse a heat pipe for heat conduction.

Heat conductivity is increased in a high temperature zone with atemperature near the temperature at the time of turn-on as the boundary.A predetermined temperature at this time is brought close to the optimumtemperature described above so that the mercury concentration of thewhole discharge tube can be substantially optimized. To obtain a desiredheat conduction performance, the present invention uses a material theboiling point of which is close to the predetermined temperaturedescribed above.

The heat discharge source is a reflector disposed around the dischargetube. It is also possible to dispose a heat radiation sheet that comesinto contact with the heat conduction member or the reflector. The heatradiation plate can be arranged in such a fashion as not to come intocontact with the heat conduction member below the predeterminedtemperature. The heat conduction member and/or a member in the proximityof the heat conduction member has a white color or is transparent tosuppress light absorption. Brightness is maintained as a leaking currentfrom the discharge tube is decreased.

A backlight of a display device according to another aspect of thepresent invention uses an interference type mirror having amulti-layered structure of a plurality of transparent film layers devoidof light absorption that is disposed below the heat conduction plate. Apart, or the whole, of the plurality of film layers have birefringence.In the film layers having birefringence, two linearly polarized lightshaving the same wavelength substantially interfere with each other, orare reflected by different layers, so that a predetermined phasedifference occurs in the reflected rays of light. The angle between thedirection of the phase advance axis or the phase delay axis of the layerof the interference type mirror having birefringence and the directionof polarized light reflected by the polarization isolation device is setto about 45 degrees (within an angular range of 23 to 67 degrees).

As a result, it is possible to obtain a liquid crystal display devicehaving high brightness in which the absorption loss and the scatteringloss in the interference type mirror do not exist, the reflection factorcan be brought to 100% (without the transmission loss), and thereflected light can more easily pass through the polarization isolationdevice.

The present invention further provides a backlight comprising adischarge tube which contains mercury and in which almost all liquidmercury other than the amount of gaseous mercury at the time ofdischarge is gathered at a first position spaced apart from the endportions of the discharge tube, and a cooling device for cooling thefirst position of the discharge tube. This backlight can be used withouta change of light emission performance even when time passes.

The present invention further provides a method of producing a backlighthaving a discharge tube containing mercury, comprising the steps ofcollecting almost all liquid mercury other than the amount of gaseousmercury at the time of discharge to a first position of the dischargetube spaced apart from the end portions of the discharge tube, and thendisposing a cooling device for cooling the first position of thedischarge tube. The backlight produced by this production method can beused without a change of light emission performance even when timepasses.

Preferably, the mercury comprises a plurality of mercury particleshaving a size of not greater than 0.2 mm or soaks into the fluorescentmaterial applied to the inner wall of the discharge tube. According tothis feature, liquid mercury collected at the first position does notmove but is held at this first position, and light emission performancedoes not change.

Preferably, the cooling device includes a cooling capacity varyingmechanism. According to this feature, light emission performance doesnot change even when the backlight is used in an environment having alarge temperature change.

Preferably, the cooling device includes a movable heat conductionmember. According to this feature, the backlight can be used without thechange of light emission performance even when it is used in anenvironment having a large temperature change.

Further, the present invention provides a display device equipped withthe backlight described above.

The present invention further provides a display device including alight source device comprising a discharge tube which contains mercuryand in which liquid mercury is collected at a first position and a lightsource device capable of cooling the first position of the dischargetube and having a variable cooling capacity, and a display deviceilluminated by the light source device. According to this displaydevice, brightness can be greatly changed by changing the currentsupplied to the discharge tube and changing the cooling capacity of thecooling device.

An optical sheet according to the present invention includes a diffusionportion having a plurality of spaced apart projections, facing to oneside and having scattering property, and valley portions positionedbetween the projections, wherein a part of the light from the valleyportion travels without coming into contact with the adjacentprojections, another part of the light from the valley portion is madeincident to the adjacent projections and is scattered by theprojections, and the light passing through the projections is scatteredby the projections and leaves the projections.

According to this construction, a part of the light from the valleyportion travels at an angle within a predetermined range to thedirection of the normal without coming into contact with adjacentprojections. Another part of the light from the valley portion andincident into the adjacent projections enters the projections, isscattered by the projections or the surface of the projections, andleaves the projections as the scattered light. A part of the scatteredlight from the projections travels at an angle within a predeterminedrange to the normal direction without coming into contact with theadjacent projections, and another part of the scattered light from theprojections is further incident into other projections and is scattered.Therefore, among the rays of light incident to the projections andscattered by the projections, the component of the light describing alarge angle to the normal direction becomes the component of the lightdescribing a small angle to the normal direction, and the outgoing lightis gradually imparted with directivity. Therefore, a broad brightnessdistribution such that the quantity of the light at the angle of thenormal direction is the greatest and the quantity of outgoing rays oflight becomes progressively smaller as the angle to the normal directionbecomes greater, can be obtained.

According to the present invention, such features can be accomplished bythe following constructions.

An optical sheet including a diffusion portion having a plurality ofspaced apart projections, facing one side and valley portions positionedbetween the projections, wherein a layer having a scattering property isdisposed on the surface of the projections.

An optical sheet including a diffusion portion having a plurality ofspaced apart projections, facing one side and having a scatteringproperty and valley portions positioned between the projections, whereineach of the projections comprises a group of a plurality of smallscattering material particles gathered together.

An optical sheet including diffusion portion having a plurality ofspaced apart portions and having portions of non-uniform refractiveindex and portions positioned between the portions of non-uniformrefractive index and having a uniform refractive index.

An optical sheet including a diffusion portion, the diffusion portionhaving a plurality of spaced apart wall members and having scatteringproperty and openings formed between the wall members, wherein the wallmember has first and second side surfaces opposing one another, and thewall member is constituted so that the ray of light is substantiallyscatter-reflected by the first and second side surfaces.

An optical sheet comprising a diffusion portion, the diffusion portionhaving a plurality of spaced apart projections, facing one side andhaving scattering property and valley portions positioned between theprojections, and a reflecting mirror.

An optical sheet including a diffusion portion having a plurality ofspaced apart portions having non-uniform refractive index and portionshaving a uniform refractive index and interposed between the portionshaving non-refractive index, wherein the diffusion portion comprises amesh having filaments and an ink containing a resin, and the mesh isburied in said ink.

Further, the present invention provides an illumination devicecomprising a light source, a light guide plate into which the light ofthe light source is made incident, and an optical sheet disposed on oneof the sides of the light conduction plate and having the featuresdescribed above.

The present invention provides further a liquid crystal display devicecomprising a light source, a light guide plate into which the light ofsaid light source is made incident, an optical sheet having the featuresdescribed above, and a liquid crystal panel.

The present invention provides a production method of an optical sheetincluding a diffusion portion having a plurality of spaced apartprojections, facing one side and having scattering property, and valleyportions positioned between the projections, the method comprising thesteps of screen-printing ink by using a mesh containing linear memberscrossing one another, and forming the diffusion portion having theprojections and the valley portions positioned between the projections.

The present invention provides an optical member comprising a sheet-likebody having a light turning region and a light guide region continuingthe light turning region, wherein the light turning region has aplurality of spaced apart portions having a non-uniform refractive indexand portions having a uniform refractive index and positioned betweenthe portions having non-uniform refractive index, and the light guideregion is a substantially transparent region.

The present invention also provides a light source device comprising adischarge tube, a reflector for reflecting a ray of light radiated fromthe discharge tube, and support members supporting the discharge tube onthe reflector. The support members is formed of a heat insulatingstructure so as to prevent a temperature drop of a portion of thedischarge tube near electrodes of the discharge tube. Or, the dischargetube is partially formed of a heat insulating structure so as to preventa temperature drop of a portion of the discharge tube near electrodes ofthe discharge tube. Or, the support members are arranged at inwardpositions from ends of electrodes of the discharge tube so as to preventa temperature drop of a portion of the discharge tube near theelectrodes of the discharge tube. Or, a heat conduction membercontacting a central portion of the discharge tube is provided, inaddition to the support members.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent from the followingdescription of the preferred embodiments, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic view showing a liquid crystal display deviceincluding a backlight according to the present invention;

FIG. 2 is a plan view of the backlight shown in FIG. 1;

FIG. 3 is a schematic sectional view showing the light source device forexplaining the principle of the present invention;

FIG. 4 is a sectional view showing the light source device according toa modified embodiment;

FIG. 5 is a sectional view showing the light source device, passingthrough the heat conduction member shown in FIG. 4;

FIG. 6 is a rear view showing the reflector shown in FIGS. 4 and 5;

FIG. 7 is a sectional view showing the light source device according toa modified embodiment;

FIG. 8 is a sectional view of the light source device of FIG. 7, passingthrough the heat conduction member;

FIG. 9 is a perspective view showing the reflector of FIGS. 7 and 8;

FIG. 10 is a plan view of the backlight including the light sourcedevice of a modified embodiment of the present invention;

FIG. 11 is a sectional view of the light source device, passing throughthe upper discharge tubes in FIG. 10;

FIG. 12 is a view showing the cooling device including the containerinto which a material for executing a cooling function by phasetransition is loaded;

FIG. 13 is a plan view of the light source device passing, through thelower discharge tubes in FIG. 10;

FIG. 14 is a sectional view of the light source device, passing throughthe lower discharge tube, in FIG. 13;

FIG. 15 is a sectional view showing the cooling device including thecontainer in which a material for executing a cooling function by phasetransition is loaded;

FIG. 16 is a view showing the cooling device including the container inwhich a material for executing a cooling function by phase transition isloaded according to a modified example of the device of FIGS. 12 and 15;

FIG. 17 is a sectional view showing the light source device of amodified embodiment of the present invention;

FIG. 18 is a sectional view showing the light source device of amodified embodiment of the present invention;

FIG. 19 is a side view of the light source device shown in FIG. 18;

FIG. 20 is a block diagram showing the control of the fan in FIGS. 18and 19;

FIG. 21 is a sectional view showing a light source device of a modifiedembodiment of the present invention;

FIG. 22 is a sectional view showing the light source device in FIG. 21;

FIG. 23 is a perspective view showing the heat conduction member inFIGS. 21 and 22;

FIG. 24 is a sectional view showing a light source device of a modifiedembodiment of the present invention;

FIG. 25 is a partial enlarged view of the light source device in FIG.24;

FIG. 26 is a view showing the light source device in FIG. 25 duringoperation;

FIG. 27 is a sectional view showing the light source device of amodified embodiment of the present invention;

FIG. 28 is a sectional view showing the light source device in FIG. 27;

FIG. 29 is a view showing the reflector of FIGS. 27 and 28 when anadhesive is dropped;

FIG. 30 is a sectional view showing the light source device of amodified embodiment of the present invention;

FIG. 31 is a sectional view showing the light source device in FIG. 30;

FIG. 32 is a sectional view showing the light source device of amodified embodiment of the present invention;

FIG. 33 is a sectional view showing the light source device of amodified embodiment of the present invention;

FIG. 34 is a schematic perspective view showing the liquid crystaldisplay device including the light source device in FIG. 33;

FIG. 35 is a diagram showing the relation between an externaltemperature of the light source device in FIG. 33 and a current of aPeltier device;

FIG. 36 is a diagram showing an example of a driving circuit of thePeltier device;

FIG. 37 is a view showing a modified embodiment of the controller inFIG. 36;

FIG. 38 is a diagram showing the relation between an ambient temperatureand a discharge tube voltage;

FIG. 39 is a sectional view showing the light source device of amodified embodiment of the present invention;

FIG. 40 is a sectional view showing the light source device in FIG. 39;

FIG. 41 is an explanatory view explaining the operation of the lightsource device in FIGS. 39 and 40;

FIG. 42 is a sectional view showing the light source device according tothe second embodiment of the present invention;

FIG. 43 is a sectional view showing the light source device in FIG. 42;

FIG. 44 is a block diagram showing the control of the fan in FIGS. 42and 43;

FIG. 45 is an explanatory view explaining the operation of the lightsource device in FIGS. 42 and 43;

FIG. 46 is a view showing the backlight of a liquid crystal displaydevice according to the third embodiment of the present invention;

FIG. 47 is a diagram showing the relation between the linearpolarization separating device and the interference type mirror;

FIG. 48 is a diagram showing the relation between the linearpolarization separating device and the interference type mirror of thebacklight of a modified embodiment of the present invention;

FIG. 49 is a view showing a construction of the interference typemirror;

FIG. 50 is a view explaining the function of the interference typemirror;

FIG. 51 is a diagram showing the relation between a wavelength and aphase difference for explaining the characteristic example 1 of theinterference type mirror;

FIG. 52 is a diagram showing the relation between a wavelength and alight quantity ratio for explaining the characteristic example 1 of theinterference type mirror;

FIG. 53 is a diagram showing the relation between a wavelength and aphase difference for explaining the characteristic example 2 of theinterference type mirror;

FIG. 54 is a diagram showing the relation between a wavelength and alight quantity ratio for explaining the characteristic example 2 of theinterference type mirror;

FIG. 55 is a diagram showing the relation between a wavelength and aphase difference for explaining the characteristic example 3 of theinterference type mirror;

FIG. 56 is a diagram showing the relation between a wavelength and alight quantity ratio for explaining the characteristic example 3 of theinterference type mirror;

FIG. 57 is a view showing the light source device of a backlightaccording to the fourth embodiment of the present invention;

FIGS. 58A to 58C are views explaining the phenomenon where thecharacteristics of the light source device changes;

FIG. 58D is a diagram showing the relation between the brightness andthe room temperature;

FIG. 59 is a view showing a production apparatus for the light sourcedevice and its production method;

FIG. 60 is a view explaining the operation of the production apparatusof the light source device shown in FIG. 59;

FIG. 61 is a view showing a modified example of the production apparatusfor the light source device in FIG. 59;

FIG. 62 is a diagram showing a mercury concentration completion timewhen the temperature of the first position of the discharge tube ischanged while the temperature of the discharge tube is kept constant;

FIG. 63 is a view showing a modified example of the production apparatusfor the light source device in FIG. 59;

FIG. 64 is a diagram showing a mercury concentration completion timewhen the temperature of the discharge tube is changed while thetemperature of the first position of the discharge tube is keptconstant;

FIG. 65 is a view showing a modified example of the production apparatusof the light source device in FIG. 59;

FIG. 66 is a view showing a modified example of the production apparatusfor the light source device in FIG. 59;

FIG. 67 is a view showing a modified example of the production apparatusfor the light source device in FIG. 59;

FIG. 68 is a view showing a modified example of the production apparatusfor the light source device in FIG. 59;

FIGS. 69A to 69C show examples of the cooling metal device shown in FIG.68;

FIGS. 70A to 70C show examples of the range of the liquid mercuryconcentration portion formed at the first position of the discharge tubewhen the cooling metal device shown in FIGS. 69A to 69C are used;

FIGS. 71A and 71B show a modified example of the production apparatusfor the light source device in FIG. 59;

FIG. 72 is a view showing an example where the concentration position ofliquid mercury and the position of a heat conduction member are arrangedsubstantially at the central portion of the discharge tube;

FIG. 73 is a diagram showing the relation between the turn-on time ofthe discharge tube and chromaticity of light emission;

FIG. 74 is a diagram showing the relation between the turn-on time ofthe discharge tube and brightness;

FIG. 75 is a view showing a modified example of the light source device;

FIG. 76 is a partial enlarged view showing a part of the discharge tubeof FIGS. 57 to 75;

FIG. 77 is a view showing the light source device similar to the oneshown in FIG. 3 when an impact test is conducted;

FIG. 78 is a diagram showing the relation between the brightness and theroom temperatures, before and after the impact test;

FIG. 79 is a diagram showing an examination result of moving ratios ofmercury particles, before and after the impact test is conducted;

FIG. 80 is a view explaining the formation of liquid mercury particlessoaking into the fluorescent material in the discharge tube;

FIG. 81 is a view showing the backlight according to the fifthembodiment of the present invention;

FIG. 82 is a diagram showing temperature characteristics and thedistribution of liquid mercury of a prior art backlight;

FIG. 83 is a diagram showing temperature characteristic and adistribution of liquid mercury of the backlight of FIG. 81;

FIG. 84 is a diagram showing temperature characteristics and adistribution of liquid mercury of the backlight of the modified exampleof FIG. 81;

FIG. 85 is a view showing the backlight according to the sixthembodiment of the present invention;

FIG. 86 is a diagram showing the relation between the gap of second andthird heat conduction members and the tube temperature;

FIG. 87 is a view showing a modified example of the optical device shownin FIG. 85;

FIG. 88 is a view showing a modified example of the optical device shownin FIG. 85;

FIG. 89 is a diagram showing the relation between a duty ratio of thetube current and the feed power to a nichrome wire in the light sourcedevice shown in FIG. 88;

FIG. 90 is a block diagram showing the power supply to the nichrome wireof the light source device shown in FIG. 88;

FIG. 91 is a view showing a modified example of the light source devicein FIG. 85;

FIG. 92 is a view showing the display device according to the fifthembodiment of the present invention;

FIG. 93 is a perspective view showing the light source device of thedisplay device in FIG. 92;

FIG. 94 is a transverse sectional view of the light source device inFIG. 93;

FIG. 95 is a side view of the discharge tube and the cooling device, asviewed from arrow Q in FIG. 94;

FIG. 96 is a transverse sectional view of the light source devicesimilar to the one shown in FIG. 94 when the bimetal extends;

FIG. 97 is a side view of the discharge tube and the cooling device, asviewed from arrow Q in FIG. 96;

FIG. 98 is a diagram showing the relation between the tube current andthe brightness of a conventional display device;

FIG. 99 is a diagram showing the relation between the temperature at thefirst position of the discharge tube and brightness;

FIG. 100 is a view showing an example of the use of the shape memoryalloy as the cooling device of the light source device of the displaydevice;

FIG. 101 is a view showing the state where the shape memory alloy inFIG. 100 is separated from the reflector;

FIG. 102 is a view showing the state where the shape memory alloy inFIG. 100 keeps contact with the reflector;

FIG. 103 is a view showing an example of the use of the shape memoryalloy and the resin as the cooling device of the light source device ofthe display device;

FIG. 104 is a view showing the state where the resin in FIG. 103 isseparated from the reflector;

FIG. 105 is a view showing the state where the resin in FIG. 103 keepscontact with the reflector;

FIGS. 106A to 106D are views showing various examples of the tube ofresin encompassing the shape memory alloy;

FIG. 107 is a view showing an example of the use of the spring and themagnet as the cooling device of the light source device of the displaydevice;

FIG. 108 is a view showing the state where the magnetic substance inFIG. 107 is separated from the electromagnet;

FIG. 109 is a view showing the state where the magnetic substance inFIG. 107 is attracted to the electromagnet;

FIG. 110 is a view showing an example of the use of the ball memberincluding the metal rod as the cooling device of the light source deviceof the display device;

FIG. 111 is a view showing the state where the end face of the metal rodof the ball member in FIG. 110 is separated from the electromagnet;

FIG. 112 is a view showing the state where the end face of the metal rodof the ball member in FIG. 110 is attracted to the electromagnet;

FIG. 113 is a view showing an example of the use of a phase transitionmember as the cooling device of the light source device of the displaydevice;

FIG. 114 is a view showing the state where the liquid in FIG. 113 movesto the low position in each capsule;

FIG. 115 is a view showing the state where the liquid in FIG. 113changes to a gas, and the gas spreads in each capsule;

FIG. 116 is a view showing the liquid crystal display device includingan optical sheet and an illumination device according to the seventhembodiment of the present invention;

FIGS. 117A to 117C are sectional views showing various examples of theoptical sheet shown in FIG. 116;

FIGS. 118A to 118C are plan views showing arrangement examples ofprojections of the optical sheet;

FIG. 119 is a view explaining the construction and operation of theoptical sheet;

FIG. 120 is a view explaining projections and valley portions of theoptical sheet;

FIG. 121 is a diagram showing a brightness distribution of the lightfrom the optical sheet;

FIG. 122 is a diagram showing a brightness distribution of the lightfrom the projections of the optical sheet;

FIG. 123 is a view showing a prism sheet;

FIG. 124 is a view showing the optical sheet according to anotherexample;

FIG. 125 is a diagram showing the brightness gain of the light of theoptical sheet in FIG. 124;

FIGS. 126A to 126C are views explaining the operation of the opticalsheet in FIG. 124;

FIGS. 127A to 127E are views explaining the operation of the opticalsheet in FIG. 124, and showing the light from several points of sidesurfaces of projections and valley portions without coming into contactwith adjacent projections;

FIGS. 128A and 128B are views showing examples of fabrication of theoptical sheet by screen-printing using a mesh;

FIG. 129 is a view showing an example of the mesh used in FIG. 128;

FIGS. 130A to 130C are views showing another example of fabrication ofthe optical sheet using the mesh;

FIGS. 131A to 131C are views showing still another example offabrication of the optical sheet using the mesh;

FIG. 132 is a diagram showing the brightness distribution of the opticalsheet produced by the method shown in FIGS. 131A to 131C;

FIG. 133 is a view showing still another example of fabrication of theoptical sheet using the mesh;

FIG. 134 is a view showing an example of the mask when producing theoptical sheet using the mask;

FIG. 135 is a view showing another example of the mask;

FIG. 136 is a view showing the projections formed by the method of FIG.134;

FIGS. 137A and 137B are views showing the engraving roll;

FIG. 137C is a view showing an example of fabrication of the opticalsheet using the engraving roll;

FIG. 138 is a view showing an application example of the optical sheetin FIG. 124;

FIG. 139 is a view showing another application example of the opticalsheet in FIG. 124;

FIG. 140 is a view showing still another application example of opticalsheet in FIG. 124;

FIG. 141 is a view showing an example of use as a reflection typeoptical sheet;

FIG. 142 is a view showing an example of the liquid crystal displaydevice in which the reflection type optical sheet is disposed below thelight guide plate;

FIGS. 143A and 143B are views showing a modified example of the liquidcrystal display device shown in FIG. 142;

FIGS. 144A and 144B are views showing a modified example of the lightconduction plate shown in FIGS. 143A and 143B;

FIG. 145 is a view showing an application example of the optical sheetin FIG. 141;

FIG. 146 is a view showing another application example of the reflectiontype optical sheet in FIG. 141;

FIG. 147 is a view showing still another application example of thereflection type optical sheet in FIG. 141;

FIGS. 148A to 148D are views showing various examples of the opticalsheet;

FIGS. 149A to 149E are views showing various examples of the opticalsheet;

FIGS. 150A to 150E are views showing various examples of the opticalsheet;

FIG. 151 is a view showing an example of the optical sheet producedusing a mesh;

FIGS. 152A and 152B are views showing a further example of the opticalsheet;

FIGS. 153A to 153C are views showing further examples of the opticalsheet;

FIG. 154 is a view showing an optical sheet according to the eighthembodiment of the present invention;

FIG. 155 is a partial enlarged sectional view of the optical sheet inFIG. 154;

FIG. 156 is a top perspective view showing a modified example of theoptical sheet in FIG. 154;

FIGS. 157A to 157D are views showing still another example of theoptical sheet in FIG. 154;

FIG. 158 is a sectional view showing the backlight according to theninth embodiment of the present invention;

FIG. 159 is a view showing a modified example of the backlight shown inFIG. 158;

FIG. 160 is a view showing a modified example of the backlight shown inFIG. 158;

FIG. 161 is a view showing a modified example of the backlight shown inFIG. 158;

FIG. 162 is a view showing a modified example of the backlight shown inFIG. 158;

FIG. 163 is a view showing a modified example of the backlight shown inFIG. 158;

FIG. 164 is a perspective view showing the notebook type personalcomputer including the light source device according to the tenthembodiment of the present invention;

FIG. 165 is a perspective view showing the monitor including the lightsource device;

FIG. 166 is a plan view of the light guide plate and the light sourcedevice of the display device of FIG. 164;

FIG. 167 is a sectional view of the light guide plate and the lightsource device of FIG. 166;

FIG. 168 is a sectional view showing the discharge tube;

FIG. 169 is a sectional view of the light source device including thedischarge tube and the reflector;

FIG. 170 is a sectional view of the light source device, taken along theline VII-VII in FIG. 169;

FIG. 171 is a sectional view of the light source device including thedischarge tube and the reflector according to another example;

FIG. 172 is a sectional view of the support member of FIG. 171;

FIG. 173 is a sectional view of the light source device including thedischarge tube and the reflector according to a further example;

FIG. 174 is a sectional view of the light source device including thedischarge tube and the reflector according to a further example; and

FIG. 175 is a sectional view of the light source device including thedischarge tube and the reflector according to a further example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be explained hereinafter withreference to the accompanying drawings. FIG. 1 illustrates a liquidcrystal display device including a backlight according to the presentinvention, and FIG. 2 is a plan view of the backlight shown in FIG. 1.Referring to FIGS. 1 and 2, the liquid crystal display device 10includes a liquid crystal panel 12 and a backlight 14. The backlight 14includes a light guide plate 16, light source devices 18 disposed oneither side of the light guide plate 16, a scatter reflection plate 20disposed below the light guide plate 16 and a scatter plate 22 disposedabove the light guide plate 16.

Each light source device 18 comprises two discharge tubes 24 and areflector 26. A part of the light from the discharge tube 24 is madedirectly incident to the light guide plate 16 and another part isreflected by the reflector 26 and is made incident to the light guideplate 16. The light travels in the light guide plate 16, is reflected bythe scatter reflection plate 20, leaves the light guide plate 16 towardsthe liquid crystal panel 12, is scattered by the scatter plate 22 and isthen made incident to the liquid crystal panel 12. The liquid crystalpanel 12 forms an image. The ray of light supplied from the backlight 14illuminates the image formed by the liquid crystal panel 12.Consequently, an observer can view a bright image.

FIG. 3 is a schematic sectional view showing the light source device 18for explaining the principle of the present invention. In thisembodiment, each discharge tube 24 is a cold cathode-ray tube called a“fluorescent lamp”. The discharge tube 24 has an inner diameter of 2.0mm, an outer diameter of 2.6 mm and a full length of 316 mm (powerconsumption: 3.5 W). Mercury 28 is contained and sealed in the dischargetube 24, and a fluorescent material 30 is coated on the inner wall ofthe discharge tube 24. The reflector 26 is an aluminum mirror and has aheight (height in the direction of the thickness of the light guideplate 16) of 8.5 cm so as to cover the two discharge tubes 24.

A heat conduction member 32 is attached to the reflector 26 so as tocontact a part of the discharge tube 24. Therefore, a part of thedischarge tube 24 is locally cooled by the heat conduction member 32.The reflector 26 is made of metal and has high heat conductivity andhigh heat radiation property, so heat of the discharge tube 24 istransferred to the reflector 26 through the heat conduction member 32and is discharged from the reflector 26.

In this way, in the present invention, a portion having a lowtemperature in the discharge tube 24 is created, and mercury 28 remainsmainly in the liquid state at the low temperature portion. Mercury 28mainly evaporates at the low temperature portion, and the evaporatedmercury gas 28G is diffused to the entire portion in the discharge tube24. The mercury gas 28G thus diffused returns also to the lowtemperature portion. In this way, the mercury gas 28G is distributedalmost uniformly in the entire discharge 24 and the pressure of themercury gas 28G is substantially uniform in the entire portion of thedischarge tube 24. In other words, the concentration of the mercury gas28G can be controlled by creating the low temperature portion in thedischarge tube 24.

The mercury gas 28G emits ultraviolet light, upon discharge, inside thedischarge tube 24. When the ultraviolet light impinges against thefluorescent material 30, the visible light leaves the discharge tube 24.The quantity of the visible ray from the discharge tube 24 issubstantially proportional to the current. The mercury gas also has aproperty of absorbing the ultraviolet rays, and the quantity of thevisible ray of light outgoing from the discharge tube 24 becomes maximumat an optimal mercury gas concentration and an optimal internaltemperature of the tube corresponding to the former. The quantity of thevisible light from the discharge tube 24 decreases from the maximumvalue both when the mercury gas concentration is higher or lower thanthe optimal value and when the internal temperature of the tube ishigher or lower than the optimal value. The present invention createsthe low temperature portion in the discharge tube 24 so that theinternal temperature of the tube is identical to, or approximatelyidentical to, the optimal value and the brightness of the ray of lightoutgoing from the discharge tube 24 becomes a maximum.

By constructing the backlight so that the heat conduction member 32 iscoupled to the discharge tube 24 and the reflector 26, the heatconduction member 32 can be arranged in a limited space within thereflector 26 covering the discharge tube 24, and heat of a part of thedischarge tube 24 can be efficiently released. The heat conductionmember 32 is preferably made of a nonmetal, and is made of at least oneof a heat conductive resin, a heat conductive rubber, and a heatconductive adhesive.

FIG. 4 is a sectional view showing the light source device of a modifiedembodiment. FIG. 5 is a sectional view of the light source device ofFIG. 4, passing through the heat conduction member. FIG. 6 is a rearview of the reflector shown in FIGS. 4 and 5. Referring to FIGS. 4 to 6,the light source device 18 includes two discharge tubes 24, a reflector26 covering two discharge tubes 24 and a heat conduction member 32keeping contact with a part of the discharge tubes 24 and attached tothe reflector 26. The discharge tubes 24 and the reflector 26 are thesame as those explained with reference to FIG. 3.

The heat conduction member 32 is made of a heat radiating silicone(SE4450 of Dow Corning Toray Silicone Corporation, heat conductivity:2.5 W/m/K) disposed at the center of the discharge tubes 24 and having awidth D (for example, about 2.0 mm). The heat conduction member 32 isfilled in the reflection 26, with such a height from the bottom of thereflection 26 that the half of the discharge tubes 24 are buried. A heatsink 34 is fitted to the portion of the back of the reflector 26corresponding to the arrangement position of the heat conduction member32. In this instance, a heat conductive adhesive transferring tape(9822, a product of Sumitomo 3M Corporation, heat conductivity: 0.61W/m/K) 36 is employed.

As a result, heat conductivity can be improved by 6.1 times incomparison with the case where the heat conduction member 32 and theheat sink 34 are not disposed. In the conventional constructions, thetemperature of the discharge tubes 24 is higher than the roomtemperature by 45° C. when rated power is supplied to the dischargetubes 24, and is around the optimal temperature (65° C.) of thedischarge tubes (cold-cathode tubes) 24 having the inner diameter of 2.0mm. In other words, when a power greater than rated power is supplied tothe discharge tubes 24, the light emission quantity is likely to drop.In contrast, this embodiment can bring the temperature of the mostcooled point of the tube surface to the optimal temperature at power of6 W. As a result, the maximum value of brightness of the light sourcecan be increased 1.7 times.

FIG. 7 is a sectional view showing the light source device of a modifiedembodiment of the present invention. FIG. 8 is a sectional view of thelight source device of FIG. 7, passing through the heat conductionmember. FIG. 9 is a perspective view showing the reflector of FIGS. 7and 8. In FIGS. 7 to 9, the light source device 18 includes twodischarge tubes 24, a reflector 26 covering two discharge tubes 24 and aheat conduction member 32 keeping contact with a part of the dischargetubes 24 and fitted to the reflector 26. The discharge tubes 24 and thereflector 26 are the same as those explained with reference to FIG. 3. Aheat sink 34 is fitted to the back of the reflector 26 using a heatconductive adhesive transfer tape 36.

The heat conduction member 32 is a heat conductive pad (4470CV, aproduct of Sumitomo 3M Corporation, heat conductivity: 2.0 W/m/K). Theheat conduction member 32 has a width D (e.g. about 1.5 mm) at thecenter of the discharge tubes 24 and the reflector 26. Two through-holeshaving a diameter of 2.0 mm are formed in the heat conduction member 32so that the discharge tubes 24 can be inserted in these through-holes.Both surfaces (front and back) to which the through-holes open arecoated with a white pigment. This coating is directed to reduce theamount of light absorbed by the heat-conductive pad (dark gray color)and to reduce heat input/output from the surface.

Each discharge tube 24 is fitted into the through-hole of the heatconduction member 32 and is set so that the heat conduction member 32 ispositioned at the center of the discharge tube 24. Since eachthrough-hole of the heat conduction member 32 is a little smaller thanthe outer diameter of the discharge tube 24, the flexible heatconduction member 32 undergoes deformation and comes into close contactwith the discharge tube 24. Thereafter the reflector 26 and thedischarge tubes 24 are combined with one another. In this instance, asilicone type adhesive is applied to eliminate the gap between thereflector 26 and the discharge tubes 24. As a result, heat conductioncan be improved 4.5 times in comparison with the prior art, and thepower for achieving the optimal temperature of the discharge tubes 24 is5.5 W. Further, the maximum light quantity becomes 1.6 times of theprior art.

FIG. 10 is a plan view showing a backlight including the light sourcedevice of the modified embodiment of the present invention. FIG. 11 is asectional view of the light source device, passing through the upperdischarge tubes of FIG. 10. FIG. 12 is a sectional view showing acooling device of FIG. 11, including a container in which a materialexhibiting a cooling function by phase transition is inserted. FIG. 13is a sectional view of the light source device, passing through thelower discharge tubes in FIG. 10. FIG. 14 is a sectional view of thelight source device, passing through the lower discharge tubes in FIG.13. FIG. 15 is a sectional view of a cooling device of FIG. 14,including a container in which a material exhibiting a cooling functionby phase transition is inserted.

In FIGS. 10 to 15, the backlight 14 includes a light guide plate 16 andtwo light source devices 18 disposed on either side of the light guideplate 16. In this case, the liquid crystal display device is used as amonitor and the backlight 14 is vertically disposed. Therefore, the twolight source devices 18 are the light source device 18 positioned aboveand the light source device 18 positioned below, as viewed in FIG. 10.

Each light source device 18 includes two discharge tubes 24, a reflector26 covering two discharge tubes 26 and a heat conduction member keepingcontact with a part of the discharge tubes 24 and fitted to thereflector 26. The discharge tubes 24 and the reflector 26 are the sameas those explained with reference to FIG. 3. Further, a cooling device(or a heat siphon device) exhibiting a cooling function by phasetransition is interposed between the heat conduction member 32 and thereflector 26.

The cooling device 38 of the light source device 18 positioned at theupper position shown in FIGS. 11 and 12, comprises a container 38 a madeof a 0.05 mm-thick stainless steel sheet and having a height of 2.5 mm,and a material 38 b exhibiting the cooling function by phase transition.After the container 38 a is exhausted, the material 38 b (methylalcohol) exhibiting the cooling function is inserted and sealed in thecontainer 38 a. The discharge tubes 24 are positioned below the coolingdevice 38 and heat the material (methyl alcohol) 38 b exhibiting thecooling function that stays at the lower side in the container 38 a.When the temperature of the discharge tubes 24 reaches the optimaltemperature (65° C.) described above, methyl alcohol boils anddrastically evaporates upward. The upper surface of the container 38 akeeps contact with the reflector 26. Upon coming into contact with thissurface, methyl alcohol vapor condenses. The resulting droplets ofmethyl alcohol return to the lower surface owning to gravity. A heatcycle is formed in this way, and heat is transferred from the dischargetubes 24 to the reflector 26.

The cooling device 38 of the light source device 18 positioned at thelower position, as shown in FIGS. 13 and 15, comprises a container 38 amade of a stainless steel sheet and a material 38 b (methyl alcohol)exhibiting the cooling function by phase transition. The cooling device38 of the light source device 18 positioned at the lower position hassubstantially the same construction and operation as the cooling device38 of the upper light source device 18. In the case of the coolingdevice 38 of the lower light source device 18, however, the reflector 26is positioned below the cooling device 38. Therefore, the upper wall ofthe container 38 a does not come into contact with the reflector 26.Hence the upper wall 38 c of the container 38 a is formed to a greatersize than the height (in the direction of height of the light conductionplate) of the reflector 26 so that the upper wall 38 c of the container38 a can be brought into contact with the sidewall of the reflector 26.

FIG. 16 is a sectional view showing a modified embodiment of the coolingdevice as shown in FIGS. 12 and 15 including the container in which thematerial exhibiting the cooling function by phase transition is fitted.The cooling device 38 of the light source device 18 includes a container38 a made of a stainless steel sheet, a material (methyl alcohol) 38 bexhibiting the cooling function and a stainless steel mesh 38 d. In thismodified embodiment, the steel mesh 38 d returns the material (methylalcohol) 38 b exhibiting the cooling function to the evaporation sideowing to capillary action. Therefore, it is not necessary to constitutethe upper and lower light source devices into separate constructions asin FIGS. 12 and 15. This has a structure similar to that of a heat pipein place of the heat siphon device in FIGS. 12 and 15.

FIG. 17 is a sectional view showing a light source device of anothermodified embodiment of the present invention. The light source device 18of this modified embodiment includes discharge tubes 24 and a reflector26 in the same way as the embodiment described above. Each dischargetube 24 is covered with a heat conductive pad (ring) 32A. An acryliccontainer 40 is interposed between the heat conductive pad 32A and thereflector 26. The container 40 has a shape corresponding to the internalshape of the reflector 26 and to the shape of the heat conductive pad32A. Glycerol 42 is fully charged into the container 40. The container40 has a width of 10 mm. Since the heat conduction member comprising theheat conductive pad 32A, the container 40 and the glycerol 42 isdisposed, heat conductivity can be improved 2.5 times that of the priorart and, eventually, the maximum brightness of the light source becomes1.2 times that of the prior art.

FIG. 18 is a sectional view showing the light source device of a furtherembodiment of the present invention. FIG. 19 is a side view of FIG. 18,and FIG. 20 is a block diagram showing the control of the fan shown inFIGS. 18 and 19. The light source device 18 of this embodiment includesdischarge tubes 24 and a reflector 26 in the same way as the modifiedembodiment described above. A heat conductive member 32B is made of heatradiating compound or a heat radiating silicone that is filled betweenthe discharge tubes 24 and the reflector 26 to the height of the half ofthe discharge tubes 24. The heat conduction member 32B is 10 mm wide. ADC fan 44 having blades of a 40 mm in diameter is fitted using a heatconductive adhesive transfer tape 36 to the arrangement position of theheat conduction member 32B spaced apart by 10 mm from the back of thereflector 26.

In FIG. 20, the fan 44 is shown connected to a DC power source 46. Apotential difference detection circuit and a fan control circuit 50 thatreceive the output of a thermocouple 48 control the fan 44. Rotationcontrol of the fan 44 is clone according to temperature as tabulatedbelow in accordance with the output of the thermocouple 48. The distalends of the thermocouple 48 are arranged in touch with the surface ofthe discharge tubes 24 at the positions at which the discharge tubes 24keep contact with the heat conductive compound and in a lower directionas to the rotating direction of the discharge tubes 24 due to thegravitational operation.

Temperature of Thermocouple(° C.) Speed of Fan (rpm)  ~45 0 46~65 3,00065~  4,000

FIG. 21 is a sectional view showing the light source device of stillfurther embodiment of the present invention. FIG. 22 is a sectional viewof the light source device in FIG. 21. FIG. 23 is a perspective viewshowing a heat conduction member in FIGS. 21 and 22. The light sourcedevice 18 of this embodiment includes discharge tubes 24, a reflector 26and a heat conduction member 32 in the same way as in the embodimentdescribed above. A heat sink is fitted to a part of the back of thereflector 26. The heat conduction member 32 is made of a heat conductiverubber having a large heat expansion coefficient. Through-holes having adiameter of 2.4 mm are bored in the heat conductive rubber. Thedischarge tubes 24 pass through the respective through-holes and arepositioned in such a manner that the heat conductive rubber exists atthe center of each discharge tube 24. The width D of the heat conductionmember 32 is 1.5 mm. A 0.5 mm-thick smoked acrylic plate (white color)52 is bonded to the surface of the heat conduction member 32 that is outof contact from the reflector 26. The smoked acrylic plate 52 operatesas a restriction plate, so that the heat conduction member 32 made ofthe heat conductive rubber can expand only in the direction facing thereflector surface (at room temperature). When the discharge tubes 24 areturned on and the heat conduction member 32 is heated to 60° C., theheat conduction member 32 comes into contact with the reflector 26. As aresult, heat conductivity is at least twice, at a temperature of 60° C.or more, of the heat conductivity in air, and maximum brightness of thelight source is 1.1 times that of the prior art.

FIG. 24 is a sectional view showing the light source device of a stillfurther modified embodiment of the present invention. FIG. 25 is apartial enlarged view of the light source device. FIG. 26 shows thelight source device of FIG. 25 in operation. The light source device 18of this embodiment includes discharge tubes 24, a reflector 26 and aheat conduction member 32 in the same way as in the foregoingembodiments. A heat sink is fitted to a part of the back of thereflector 26. The heat conduction member 32 uses a heat conductive pad(4470CV, a product of Sumitomo 3M Corporation, heat conductivity: 2.0W/m/K). This heat conduction member 32 is shaped into a thickness of 1.5mm so that the heat conduction member 32 comes into contact with bothdischarge tubes 24 and reflector 26. A bimetal 54 is fitted to thereflector 26 on both sides of the heat conduction member 32. When thetemperature of the discharge tubes 24 is lower than a predeterminedvalue, deformation of the bimetal is small and the heat conductionmember 32 substantially does not come into contact with the dischargetubes 24. When the temperature of the discharge tubes 24 becomes higherthan the predetermined value, deformation of the bimetal becomes sogreat that the heat conduction member substantially comes into contactwith the discharge tubes 24 and cools a part of the discharge tubes 24.

FIG. 27 is a sectional view showing the light source device of a stillfurther embodiment of the present invention. FIG. 28 is a sectional viewof the light source device shown in FIG. 27. FIG. 29 shows a reflectorwhen an adhesive is dropped. The light source device 18 in thisembodiment includes discharge tubes 24, a reflector 26 and a heatconduction member 32 in the same way as in the foregoing embodiment. Theheat conduction member 32 uses a heat conductive adhesive (SE4486, aproduct of Dow Corning Toray Silicone Corporation). As shown in FIG. 29,the adhesive is dropped and applied onto the inner surface of thereflector 26 to form a ball of the adhesive having a width of 5.8 mm anda height of 0.9 mm. Each discharge tube 24 is urged, toward the adhesiveball, by a distance of 0.8 mm from the inner surface of the reflector26.

FIG. 30 is a sectional view showing the light source device of a stillfurther modified embodiment of the present invention. FIG. 31 is asectional view of the light source device shown in FIG. 30. The lightsource device 18 in this embodiment includes discharge tubes 24, areflector 26 and a heat conduction member 32 made of a heat conductiveadhesive in the same way as the foregoing embodiment. The heatconduction member 32 is sandwiched by a translucent silicone rubbers(TSE221-5U, a product of GE-Toshiba Silicone Corporation) 52 a. Thesilicone rubber 52 a has holes through which the discharge tubes 24 arepassed. The discharge tubes 24 are passed through a pair of holes of thesilicone rubber 52 a, and 0.02 ml of the adhesive is charged into aspace between the pair of silicone rubbers 52 a while this space ismaintained. As a result, the heat conduction member 32 covers ⅓ of thesurface (circumference) of the discharge tubes 24 and is fitted to thereflector 26. In this embodiment, the heat radiation property to thereflector 26 is improved 1.8 times the prior art device. When thecurrent to the discharge tubes 24 is set to 10 mA, the temperature riseof the discharge tubes 24 can be limited to about 20° C. with respect toroom temperature.

FIG. 32 is a sectional view showing the light source device of a yetfurther embodiment of the present invention. The light source device 18in this embodiment includes discharge tubes 24, a reflector 26 and aheat conduction member 32 made of a heat conductive adhesive in the sameway as the foregoing embodiment. The heat conduction member 32 comprisesa heat conductive rubber 32C and a heat conductive adhesive. To form theheat conductive rubber 32C, a 0.5 mm-thick silicone type heat conductiverubber (HT-50, a product of Nittoh Shinko Corporation) is cut to 5.5mm×5.0 mm and is bonded to the discharge tubes 24 and the reflector 26by using the heat conductive adhesive 32D. The heat conductive adhesive32D is applied to a wire having a diameter of 0.5 mm and is addeddrop-wise onto the heat conductive rubber 32C. After the heat conductiverubber 32C is spread on the entire surface the heat conductive rubber32C, the heat conductive rubber 32C is bonded to the discharge tubes 24and the reflector 26. As a result, heat radiation efficiency to thereflector can be improved 1.9 times in comparison with the conventionalstructure. When the current to the discharge tubes 24 is 10 mA, thetemperature rise of the discharge tubes 24 is about 18° C. with respectto the room temperature.

FIG. 33 is a sectional view showing the light source device of a yetfurther embodiment of the present invention. FIG. 34 is a schematicperspective view showing a liquid crystal display device including thelight source device of FIG. 33. FIG. 35 is a graph showing the relationbetween the external temperature of the light source device of FIG. 33and the current of the Peltier element. FIG. 36 is a block diagramshowing an example of a driving circuit for the Peltier element. Thelight source device 18 in this embodiment includes discharge tubes 24, areflector 26 and a heat conduction member 32 made of a heat conductiveadhesive in the same way as the foregoing embodiment. The heatconduction member 32 comprises a heat conductive rubber 32C and a heatconductive adhesive.

A hole having a size of 5.5 mm×5.5 mm is bored in the bottom of thereflector 26. A heat conduction member 32 made of a heat conductiverubber (Thercon GR-D, a product of Fuji Polymer Corporation, 1.0 mmthick, heat conductivity: 1.5 W/m/K) is brought into contact with thedischarge tubes 24 on one hand and is passed through the hole of thereflector 26, on the other. A Peltier element 56 having a size of 6.0mm×6.0 mm is fitted to the outer surface of the heat conduction member32 through bonding power of the heat conductive rubber. While thePeltier element 56 and the heat conduction member 32 are pushed onto thedischarge tubes 24 at a pressure of 100 kPa, the Peltier element 56 andthe heat conduction member 32 are fixed to the reflector 26. Further, aheat sink 58 is fitted to the outside of the Peltier element 56. ThePeltier element 56 keeps contact with, or is bonded to, the reflector 26in the proximity of the point where the reflector 26 keeps contact with,or is bonded to, the heat conduction member 32.

A controller 57 controls the Peltier element 56. A DC current issupplied, by a lead wire 56 a, to the Peltier device 56. The controller57 includes a DC power source 57 a and a converting circuit 57 b, andthe output of a thermocouple 48 is supplied to the converting circuit 57b. The supply of the DC current to the Peltier element 56 is controlledwith respect to the external temperature of the backlight unit.Temperature measuring terminals of the thermocouple 48 are arranged atpositions spaced apart by 10 mm from the back (the surface notpermitting the light to leave therefrom) inside the casing of the liquidcrystal display device. Inversion of polarity at the externaltemperature of 20° C. is inhibited, and 1.2 KW is supplied at anexternal temperature of 35° C. or more. As the Peltier element 56 isdisposed, the temperature of the discharge tubes 24 can be reduced bymaximum 35° C. Incidentally, the temperature measuring terminals of thethermocouple 48 are arranged inside the casing of the liquid crystaldisplay device in this embodiment. When any correlation exists betweenthe temperature of air encompassing the discharge tubes and that of openair, the temperature measuring terminals of the thermocouple 48 may wellbe disposed at any position (outside the casing of the liquid crystaldisplay device, for example).

FIG. 37 shows a modified example of the controller shown in FIG. 36.FIG. 38 is a graph showing the relation between the ambient temperatureand the discharge tube voltage. Referring to FIG. 37, the light sourcedevice 18 in this embodiment includes discharge tubes 24, a reflector 26and a heat conduction member 32 in the same way as the foregoingembodiment. A Peltier element 56 is fitted to the outer surface of theheat conduction member 32, and a heat sink 58 is fitted to the outsideof the Peltier element 56. A controller 60 controls the Peltier element56.

The controller 60 includes a discharge tube inverter 60A for supplyingthe current to the discharge tubes 24. The inverter 60A is connected toa power source circuit 60B and a switch 60C. A timer 60D is connected tothe switch 60C. The Peltier element 56 is connected to a constantvoltage power source 60E. A voltage meter 60F is connected to theconstant voltage power source 60E to detect a voltage across bothterminals (A-A′ voltage) of the discharge tube 24. A switch 60G isdisposed in the circuit of the voltage meter 60F.

The inverter 60A and the DC power source circuit 60B supply a constantcurrent to the discharge tubes 24 to keep the light quantity of thedischarge tubes 24 substantially constant. Since the temperature of thedischarge tube 24 and its resistance have a negative correlation, thevoltage of the discharge tube 24 monotonously decreases when the currentof the discharge tube 24 is kept constant. When the current of thedischarge tube 24 is 10 mA, its voltage becomes 550 V under thetemperature condition in which the light quantity becomes maximal (seeFIG. 38).

Therefore, the controller 60 of the Peltier element 56 includes amechanism for reflecting the voltage of the discharge tube 24 andoptimizes the temperature of the discharge tube 24. The Peltier element56 controls the temperature of the discharge tube 24. The voltage meter60F measures the voltage drop of the discharge tube 24, and the timer60D controls the timing at which the voltage meter 60F measures thevoltage. The output of the voltage meter 60F is sent to the constantvoltage source 60E to reflect the voltage to the Peltier element 56.After the discharge tubes 24 are turned ON for 1 minute, the timer 60Dcuts the circuit between the HI side electrode (driving electrode) ofthe discharge tube 24 and the terminal of the voltage meter 60F. In thisway, the initial voltage (>1,000 V) at the ON time is prevented frombeing applied to the voltage meter 60F.

FIG. 39 is a sectional view showing the light source device of a stillfurther embodiment of the present invention. FIG. 40 is a sectional viewof the light source device shown in FIG. 39. FIG. 41 explains theoperation of the light source device shown in FIGS. 39 and 40. The lightsource device 18 of this embodiment includes discharge tubes 24, areflector 26, a heat conduction member 32 made of a heat-conductiverubber and a Peltier element 56 in the same way as in the foregoingembodiment. The heat conduction member 32 comprises a heat-conductiverubber 32C and a heat conductive adhesive 32D. A 0.5 mm-thick bakeliteplate 62 is bonded to the periphery of the heat conduction member 32.

Referring to FIG. 41, the curve TO represents the temperature of theheat conduction member 32. The curve T1 represents the ambient airtemperature. The curve T2 represents the difference between the curve TOand the curve T1. The position PO represents the surface of thedischarge tube 24. The position P1 represents the surface of the Peltierelement 56 on the side of the discharge tube 24. The position P2represents the surface of the Peltier element 56 on the heat dischargeside. When the Peltier element 56 absorbs much heat from the heatconduction member 32, it takes away heat from around the heat conductionmember 32 through the heat conduction member 32. In other words, heat inthe heat conduction member 32 is lost to the extent corresponding to thetemperature T2. When this heat absorption is examined, 0.7 W is takenfrom air among calorie of 1 W (per 0.36 cm²) discharged from the Peltierelement 56. In this embodiment, the bakelite plate 62 is disposed sothat heat cannot be taken away easily from the portions other than thedischarge tube 24. Consequently, cooling efficiency of the predeterminedportion of the discharge tube 24 can be drastically improved. In thisway, electric power necessary for lowering the tubular wall temperatureof the discharge tube 24 by 30° C. can be reduced from 1.2 W to 0.7 W.

FIG. 42 is a sectional view showing the light source device of thesecond embodiment of the present invention. FIG. 43 is a sectional viewof the light source device shown in FIG. 42. FIG. 44 is a circuitdiagram showing a fan controlling circuit in FIGS. 42 and 43. FIG. 45explains the operation of the light source device shown in FIGS. 42 and43. The light source device 18 in this embodiment includes a pluralityof discharge tubes 24 and a reflector 26 covering all these dischargetubes 24. A fan 64 and a duct 66 are disposed to cool the dischargetubes 24. A hole 26 a having a diameter of 0.5 mm is formed at thecenter of the bottom of the reflector 26.

Cooling air flows from the axial-flow fan (40 mm in diameter) 64 fittedbelow the reflector 26 through the duct 66 and the hole 26 a of thereflector 26 and is blown to the opposing portions of the two dischargetubes 24. As a result, one point of the mutually opposing sides of thetwo discharge tubes 24 becomes the most cooled portion as to thecircumferential direction of the discharge tubes 24 so thatnon-evaporated mercury particles 28 can be concentrated on this portionof the discharge tube 24. Since the mercury particles 28 cut off thelight, luminance drops at this portion of the discharge tube 24, but thelight outgoing from this portion of the discharge tube 24, to which themercury particles 28 adhere, are made incident to the opposing dischargetube 24. In consequence, the loss of the light quantity does not occur,and the loss of the light quantity can be substantially reduced incomparison with the case where the mercury particles are allowed toadhere to other portions. Further, the speed of the fan 64 is controlleddepending on the surface temperature of the discharge tube 24.

AS shown in FIG. 43, a thermocouple 48 is fitted to the surface of thedischarge tube 24 (at a position opposing the bottom of the reflector 26and spaced apart by 50 mm from the position at which cooling air isblown in the longitudinal direction). FIG. 44 shows a DC power source 67and a voltage converting circuit 68 for controlling the fan 64. When thetemperature detected by the thermocouple 48 is not higher than 65° C.,the speed of the fan is set to 0. The speed is controlled, depending onthe temperature, at a temperature higher than 65° C.

FIG. 46 shows the backlight of the liquid crystal display deviceaccording to the third embodiment of the present invention. Referring toFIG. 46, the backlight 70 of the liquid crystal display device includesa light guide plate 72, light source devices 74 disposed on either sideof the light guide plate 72, an interference type mirror 76 disposedbelow the light guide plate 72 and a linear polarization separatingelement 78 disposed above the light guide plate 16. Each light sourcedevice 74 comprises discharge tubes and a reflector as described above.A conventional scattering layer is coated onto the acrylic light guideplate 72 by screen-printing.

The polarization separating element 78 comprises a cholesteric liquidcrystal polymer film and broad-band 4/1 wavelength plates bonded to bothsurfaces of the cholesteric liquid crystal polymer film. Theinterference type mirror 76 has a multi-layered structure fabricated bylaminating a plurality of transparent film layers having a lightabsorption property and birefringence.

The polarization separating element 78 receives the ray of lightoutgoing from the light guide plate 16, and allows a first linearlypolarized light having a plane of polarization (vibration plane)inclusive of the transmission axis to transmit therethrough and to be asecond linearly polarized light having a plane of polarization inclusiveof the reflection axis to be reflected. The interference type mirror 76rotates the plane of polarization of the second linearly polarized lightreflected by the polarization separating element 78 and converts itmainly into the first linearly polarized light. First linearly polarizedlight so converted is made incident again to the polarization separationelement 78 through the light guide plate 72 and is transmitted throughthe polarization separating element 78. Therefore, this embodiment canimprove the utilization efficiency of light.

FIG. 47 shows the relation between the polarization separating element78 and the interference type mirror 76 shown in FIG. 46. Arrow Xrepresents the vibrating direction of the second linearly polarizedlight reflected by the polarization separating element 78. Straight lineY of solid line represents the direction of a fast axis or a slow axisof the interference type mirror 76. The straight broken line Zrepresents the fast axis or the slow axis of the ¼ wavelength plate ofthe polarization separating element 78 on the side of the light guideplate 72.

In this embodiment, the direction of the fast axis or the slow axis ofthe interference type mirror 76 represented by the line Y is arranged atan angle of 45 degrees to the vibrating direction of second linearlypolarized light reflected by the polarization separating element 78represented by the arrow X. The direction of the fast axis or the slowaxis of the ¼ wavelength plate of the polarization separating element 78represented by the line Z on the side of the light guide plate 72, too,is arranged at the angle of 45 degrees to the vibrating direction of thesecond linearly polarized light. In other words, the direction of thefast axis or the slow axis of the interference type mirror 76 is thesame as the direction of the fast axis or the slow axis of the ¼wavelength plate of the polarization separating element 78 on the sideof the light guide plate 72.

FIG. 48 shows the relation between the polarization separating element78 and the interference type mirror 76 of the backlight of anotherembodiment of the present invention. The backlight of this modifiedembodiment has the same structure as that of the backlight shown in FIG.46. In this embodiment, however, the polarization separating element 78uses the same interference type film as that of the interference typemirror 76. The interference type mirror 76 has a multi-layered structuremade of a transparent material having no light absorbing property.

Referring to FIG. 48, the fast axis or the slow axis of the interferencetype mirror 76 represented by straight line Y is arranged at an angle of45 degrees to the vibrating direction of the second linearly polarizedlight reflected by the polarization separating element 78 represented byarrow X. The direction of the fast axis or the slow axis of thepolarization separating element 78 represented by line Z is arranged atan angle of 90 degrees to the vibrating direction of the second linearlypolarized light. In other words, the direction of the fast axis or theslow axis of the interference type mirror 76 is arranged at an angle of45 degrees to the direction of the fast axis or the slow axis of thepolarization isolation element 78.

FIG. 49 shows the construction of the interference type mirror 76. Theinterference type mirror 76 is a multi-layered structure film fabricatedby alternately laminating extremely thin polyester films havingbirefringence realized by a relatively strong stretching and extremelythin polyester films having birefringence realized by a relatively weakstretching. Symbols Fi1 to Fi4 represent the film layers. Symbols di1 todi4 represent the thickness of the film layers Fi1 to Fi4.

The number “i” film layer Fi1 of the interference type mirror 76 hassuch a property that with respect to two orthogonal linearly polarizedlights “a” and “b”, a predetermined wavelength λai satisfies therelation, nai×di=nai×(N+0.5) and a predetermined wavelength λbisatisfies the relation, nbi×di+nbi×(N+0.5), where nai and nbi arebirefringence of the film for two orthogonal linearly polarized lights,and λai≈λbi.

The wavelengths λai and λbi for two linearly polarized lights “a” and““b” become approximately greater as the number “i” increases, and areintended to cover the visible light range (wavelength band: 400 to 700nm: see FIG. 50). In FIG. 50, for example, the position at which thelinearly polarized light “a” of a wavelength band of B color band isreflected (that is, the film layer) is different from the position atwhich the linearly polarized light “b” of a wavelength of B color (thatis, the film layer) is reflected. This also holds true of the linearlypolarized light of other colors.

Since the effective reflection film layers for the same wavelength λ aredifferent, a phase difference Δλ≈0.5λ occurs between two linearlypolarized light “a” and “b” having the same wavelength λ. The phasedifference preferably satisfies the relation Δλ=0.5λ at all thewavelengths but the relation Δλ≈0.5λ may well be satisfied atwavelengths of predetermined bands (420 to 500 nm in the B color band,415 to 590 nm of a G color band and 600 to 670 nm in a R color band). Aslong as Δλ or the mean value of Δλ substantially falls within the rangeof 0.25λ to 0.75λ at wavelengths of predetermined bands (420 to 500 nmin the B color band, 410 to 590 nm in the G color band and 600 to 670 nmin the R color band), the major proportion of the rays of lightreflected by the interference type mirror 76 and made incident to thepolarization separating element 78 transmit through the polarizationseparating element 78 and are effectively utilized.

FIGS. 51 and 52 show a characteristic example (1) of the interferencetype mirror 76. This example is directed to allow the B color reflectedlight and the R color reflected light to pass through the polarizationseparating element 78 to the greatest possible amount to acquire highbrightness.

FIGS. 53 and 54 show a characteristic example (2) of the interferencetype mirror 76, which is directed so that portions of spectrum existingat boundaries between three primary color portions cannot easily passthrough the polarization separating element 78 to acquire highly purechromaticity of the three primary colors.

FIGS. 55 and 56 show a characteristic example (3) of the interferencetype mirror film. This example indicates that, in the relation betweenthe fast axis (slow axis) of the interference type mirror 76 and thefast axis (slow axis) of the ¼ wavelength plate constituting thepolarization separating element 78 on the light guide plate side, thequantity of effective reflected light having a plane of polarization inthe transmitting direction of the polarization separating element 78 isgreat when both of them face the same direction but the quantity ofeffective reflected light becomes extremely small when both describe anangle of 45 degrees.

In this embodiment, light utilization efficiency can be improved by 0 to10% in the backlight 70 equipped with the polarization separatingelement 78, by replacing the conventional scatter reflection plate withthe interference type mirror 76, since scattering due to the scatterreflection plate disappears. Alternatively, by replacing theconventional metal mirror with the interference mirror 76, absorptiondue to the metal mirror does not exist, so that light utilizationefficiency can be improved by 0 to 20%. Further, when the direction ofthe fast axis (slow axis) of the birefringence film layer constitutingthe interference type mirror 76 is controlled, light utilizationefficiency can be improved by 10 to 20%.

As described above, the interference type mirror 76 is made of abirefringence film material, so that two linearly polarized lightshaving identical wavelength interfere with each other or are reflectedby different layers to impart a predetermined phase difference to thereflection type mirror 76. Furthermore, the direction of the fast axisor the slow axis of the birefringence layer of the interference typemirror 76 and the direction of polarized light reflected by thepolarization separating element 78 are set to form approximately 45degrees, so that the light quantity of two polarized lights are equal toeach other. In this case, the direction of the fast axis or the slowaxis of the birefringence layer of the interference type mirror 76 andthe direction of polarized light reflected by the polarizationseparating element 78 are set to 45°±22.50° (23 to 67°) and, by sodoing, the initial object can be accomplished.

Incidentally, the interference type mirror 76 comprises a multi-layeredstructure having a plurality of film layers, and there may be a casewhere the directions of the fast axes or the slow axes of thebirefringence layers of the interference type mirror 76 are notperfectly aligned with a predetermined direction. It can be said,however, that the directions of the fast axes or the slow axes of thebirefringence layers of the interference type mirror 76 are generallyaligned in all the film layers (or in almost all the film layers or inat least two film layers). Therefore, the directions of the fast axes orthe slow axes of the interference type mirror 76 can be determined as awhole as the mean value in the direction of the fast axes or the slowaxes of all the film layers.

More specifically, the directions of the fast axes or the slow axes ofthe birefringence layers of the interference type mirror 76 can be saidas a group of directions in which the difference between refractiveindices of the adjacent film layers taken within respective layer planesand in the same directions becomes maximum, or statistic direction (thedirection having higher correlation) of a group of the directions of thefast axes or the slow axes of the birefringence layers. The direction ofpolarized light reflected by the polarization separating element 78 canbe said as the direction of the reflecting axis of the polarizationseparating element 78.

As the interference type mirror 76 is employed, the reflection ratio ofa predetermined linearly polarized light can be improved to 100%(without the transmission loss) and the rays of reflected light can moreeasily pass through the polarization isolation device 78, since theabsorption loss and the scattering loss of the interference type mirror76 do not exist.

The construction shown in FIG. 48 can acquire the same function andeffect as described above.

Since the angle described above may be set to 45°±22.5° (23° to 67°),the processing can be done easily by using practical stretched films. Inthe practical stretched films (particularly in biaxially stretchedfilms), the fast axis (slow axis) is greatly curved in the width-wisedirection of the film roll, but this curve falls within the range of, atmost, ±20°, and therefore, no problem presumably occurs even when filmlayers of a large size are laminated as such and are then cut to producea multi-layered film.

This embodiment uses polyester as the film material of the interferencetype mirror 76, but other transparent plastic films having birefringence(such as polyethylene terephthalate) can be used. This also holds trueof the interference type polarization separating element 78.

Further, in the modified example of the interference type mirror 76,when the interference type mirror 76 has the multi-layered structure offilms having birefringence or when the interference type mirror 76 has amulti-layered structure of films having birefringence and films nothaving birefringence, the group of the directions in which thedifference between refractive indices of the birefringence layers in thesame direction becomes maximum, or the group of the directions of thefast axis or the slow axis of the birefringence layer, may have thestructure not having statistic directivity (distributed substantiallyuniformly in all directions, or directivity has no correlation).

FIG. 57 shows the light source device 18 of the backlight according tothe fourth embodiment of the present invention. The light source device18 includes a discharge tube 24 containing mercury 28, a reflector 26and a heat conduction member 32 interposed between the discharge tube 24and the reflector 26. The discharge tube has electrodes 25. Afluorescent material 30 is coated to the inner surface of a glass tubeconstituting the discharge tube 24. The heat conduction member 32 is acooling device defining a first position of the discharge tube 24 as amost cooled portion (the position of the discharge tube 24 at which theheat conduction member 32 is disposed is called the first position). Thebacklight is arranged such that liquid mercury 28 is collected at thefirst position in the discharge tube 24, and the discharge tube 24 emitsthe light with the highest brightness based on the temperature of thefirst position. In this embodiment, liquid mercury 28 is gathered at thefirst position in the discharge tube 24 through a special process beforeshipment of products. Incidentally, the light source device 18 ismounted to a housing 14H of the backlight 14 of the liquid crystaldisplay device 10.

FIGS. 58A to 58D are views explaining the change of the characteristicsof the light source device 18 when it is used while mercury is notgathered at the most cooled portion in the light source devices 18 shownin FIGS. 1 to 45. FIGS. 58A to 58C show that the position of liquidmercury 28 changes with the passage of the number of days of using thelight source device 18. FIG. 58D shows the relation between the roomtemperature and brightness.

In FIG. 58D, curve X represents the relation between the roomtemperature and brightness when the light source device 18 is usedimmediately after production. As represented by the curve X, the maximumvalue of brightness reaches the point A when the room temperature isaround 25° C., and brightness drops from the maximum value A when thetemperature changes both up and down from 25° C. Therefore, this lightsource device 18 is suitably used at the room temperature of around 25°C.

It has been found, however, that, as the number of days of using thelight source device 18 increases, the characteristics of the lightsource device 18 change. Curve Y is a graph representing the relationbetween the room temperature and brightness when the light source device18 is used for 50 days after the production. As represented by the curveY, the brightness reaches maximum at a room temperature of around 40° C.Curve Z is a graph representing the relation between the roomtemperature and brightness when the light source device 18 is used for100 days after production. As represented by the curve Z, brightnessreaches maximum when the room temperature is around 50° C. After thepassage of 100 days, the characteristics of the light source device 18do not change much but remain stable.

If the characteristics of the light source device 18 change as describedabove, the maximum brightness A cannot be achieved after the passage ofa considerable number of days when the light source device 18 is alwaysused at the room temperature, for example. In other words, brightnessaround 25° C. in the curve Y is the point B whereas brightness around25° C. in the curve Z is the point C. Thus, brightness of the lightsource device 18 drops. Therefore, it is necessary to conduct aging inwhich the light source device 18 is kept ON before shipment, tostabilize the characteristics of the light source device 18. However, ifaging is carried out for a long time, the number of production stepsincreases, a large space becomes necessary, the production costincreases, the light source device itself becomes like a used article,and brightness drops. Therefore, a light source device not calling forsuch aging and a production method of a backlight have been desired.

FIG. 58A shows the case where the light source device 18 is usedimmediately after its production. FIG. 58B shows the case where thelight source 18 is used after the passage of several days from itsproduction. FIG. 58C shows the case where the light source device 18 isused after the number of days further increases from its production. InFIGS. 58A to 58C, reference numerals 24A, 24B and 24C denote theposition in the discharge tube 24, and symbols Ta, Tb and Tc denote thetemperatures at these positions 24A, 24B and 24C, respectively. Here,Ta, Tb and Tc satisfy the relation Ta>Tb>Tc. In other words, thetemperature Tc at the position 24C in the discharge tube 24corresponding to the heat conduction member 32 is the lowest. Pa, Pb andPc represent the saturation vapor pressure of mercury at thetemperatures Ta, Tb and Tc, and satisfy the relation Pa>Pb>Pc.

Referring to FIG. 58A, liquid mercury 28 is distributed throughout thedischarge tube 24 at the initial stage of use of the light source device18. When electric power is supplied to the electrodes 25 of thedischarge tube 24, discharge starts occurring, and the temperature ofthe discharge tube 24 rises, so that the mercury gas 28G generatesultraviolet light during discharge, and the ultraviolet light impingesagainst the fluorescent material 30 to generate visible light. Thehigher the temperature Thg of mercury, the higher becomes the saturationvapor pressure Phg of mercury as expressed by the following equation(where E and k are constants).Phg=Eexp(−k/Thg)

Liquid mercury at the high temperature position 24A in the dischargetube 24 first evaporates in accordance with the saturation vaporpressure Pa that is determined by the temperature Ta. The pressure ofmercury reaches mostly the pressure Pa in all portions of the dischargetube because the mercury vapor attempts to establish pressureequilibrium. It will be assumed that the thermal gradient of thedischarge tube 24 is designed so that the temperature Ta at the position24A is 65° C. and the room temperature is 25° C. Then, the maximumbrightness is given at the temperature of 65° C. of the discharge tube24, that is, at a room temperature of 25° C. The curve X in FIG. 58Drepresents the relation between the temperature and brightness of thelight source device 18 at this situation.

In this time, the saturation vapor pressures of mercury at otherpositions 24B and 24C of the discharge tube 24 are Pb and Pc. Therefore,the mercury vapor generated at position 24A and flows in the tube isliquefied at positions 24B and 24C. This phenomenon lasts until liquidmercury no longer exists at position 24A.

In FIG. 58B, after liquid mercury no longer exists at position 24,liquid mercury at position 24B having the second highest temperatureevaporates in accordance with the saturation vapor pressure of mercuryin accordance with the temperature Tb at position 24B. In this case too,the mercury vapor attempts to establish pressure equilibrium and thepressure throughout the whole tube substantially reaches Pb. At thistime, the maximum brightness is attained at the temperature Tb of about65° C. The room temperature corresponds to 40° C. when the temperatureat the position 24B is 65° C., according to the condition of the thermalgradient described above where the temperature Ta at position 24A is 65°C. and room temperature is 25° C. The curve Y in FIG. 58D represents therelation between the temperature and luminance of the light sourcedevice 18 at this situation.

In this time, at the other position 24C of the discharge tube 24, thesaturation vapor pressure of mercury is Pc. Therefore, the mercury vaporgenerated at position 24B and spreading in the discharge tube 24 isliquefied at position 24C. This phenomenon lasts until liquid mercury nolonger exists at position 24B. Since the pressure relation is Pa>Pb,liquefaction of mercury does not occur at position 24A.

In FIG. 58C, after liquid mercury no longer exists at the position 24B,liquid mercury at position 24C as the lowest temperature positionevaporates with the saturation vapor pressure of mercury in accordancewith the temperature T_(C) at that position. In this case too, themercury vapor attempts to establish pressure equilibrium, and thepressure in the whole discharge tube substantially reaches Pc. Themaximum brightness is reached at this time when the temperature Tc isaround 65° C. The room temperature at this time corresponds to 50° C.The curve Z in FIG. 58D represents the relation between the temperatureand brightness of the light source device 18.

At position 24C having a low pressure, when the vapor pressure ofmercury in the discharge tube 24 reaches the supersaturated pressure dueto the change of the room temperatures or the like, re-liquefactionoccurs at the position 24C. In consequence, liquid mercury positioned atthe position 24C does not move and the temperature-brightnesscharacteristics are stabilized.

Therefore, when the light source 18 is shipped as the product, it isdesired that liquid mercury 24 be collected from the beginning at theposition (first position) corresponding to the heat conduction member 32in the discharge tube 24 by a method not relying on aging, and then toarrange the heat conduction member 32 at the first position so that thelight source device 18 can be used at desired brightness.

FIG. 59 shows an apparatus and a method of fabricating the light source18. At this stage, the discharge tube 24 that has already been producedis employed, and liquid mercury is collected at the first position (theposition corresponding to the heat conduction member 32) of thedischarge tube 24. The reflector 26 and the heat conduction member 32are not yet fitted to the discharge tube 24.

The production apparatus of the light source device 18 includes anelectric furnace 80 having a heater 82 and a cooling opening 84. Acooling fan 86 is disposed in a duct 88 fitted in the cooling opening84. The cooling fan 86 blows cooling air to the first position of thedischarge tube 24 in the electric furnace 80.

The discharge tube 24 is a cold-cathode tube called a “fluorescent lamp”that is the same as the one explained with reference to FIGS. 1 to 3.The discharge tube 24 has, for example, an inner diameter of 2.2 mm, anouter diameter of 2.8 mm and a full length of 386 mm (consumed power:3.5 W). The inner volume of the discharge tube 24 is 1,467 mm³, and 2.5mg of mercury 28 is charged in the discharge tube 24. A fluorescentmaterial 30 (not shown in FIG. 59) is applied to the inner wall of theglass tube forming the discharge tube 24. Electrodes 25 are fitted toboth ends of the discharge tube 24, and mercury and a rare gas arecharged into the discharge tube 24.

FIG. 60 explains the operation of the production apparatus of the lightsource device 18 shown in FIG. 59. The discharge tube 24 is placed intothe electric furnace 80 and electric power is applied to the heater 82to raise the temperature in the electric furnace 80. First, liquidmercury 28 is distributed throughout in the discharge tube 24, asexplained with reference to FIG. 48(A). As the temperature in theelectric furnace 80 rises, liquid mercury 28 starts evaporating.

Preferably, the temperature in the electric furnace 80 is raised to 300°C. or higher than 300° C. When the temperature in the discharge tube 24reaches 300° C. or above, the saturation vapor pressure of mercury inthe discharge tube 24 becomes high and all mercury charged evaporates.The temperature in the electric furnace 80 is raised preferably to 350°C. or above. In this embodiment, the temperature in the electric furnace80 is raised to 400° C.

The reason why the temperature in the electric furnace 80 is raised to300° C. or above is as follows. When 2.5 mg of mercury is charged in thedischarge tube 24 having the volume of 1,467 mm³, the amount of mercurythat can evaporate in the discharge tube 24 is calculated in thefollowing way. First, the saturation vapor pressure P (Torr) of mercuryis given by P=Eexp (A/T). Here, T is the temperature (K), E is aconstant (=1.51×10⁸) and A is a constant (=7,495). The saturation vaporpressure P′ expressed in terms of Pascal is P′ (Pa)=133.32×P.

When this value is put into the equation of state of a gas (PV=nRT) andthe amount of mercury capable of evaporating in the discharge tube 24 iscalculated, the result is tabulated in Table 1. R is a constant (=8.314)and n=N/200.6. The numeric value 200.6 is the atomic weight of mercury,and the evaporated mercury amount N is given by N=PV×200.6/R/T.

TABLE 1 Relation between Temperature and Amount of Mercury capable ofevaporation Temp. (C.) Temp. (K) P′ (Pa) Evaporated Hg (N: mg) 0 2730.024025 0.0000 50 323 1.684061 0.0002 100 373 37.77732 0.0036 150 423406.2085 0.034 200 473 2643.547 0.20 250 523 12025.19 0.81 300 57341993.22 2.6 350 623 119975.8 6.8 400 673 293267.8 15.4 450 723 633499.431.0 500 773 1238675 56.7

It can be understood from Table 1 that when the temperature is 300° C.or above, 2.5 mg of mercury can completely evaporate. However, since theamount of mercury charged in the discharge tube 24 has variance, thetemperature is preferably 350° C. or above. Even when variance of 0.5 mgexists, mercury can completely evaporate if the temperature is 350° C.or above.

In the embodiment shown in FIG. 60, the discharge tube 24 is heatedfirst while the cooling fan 86 is kept stopped. When the temperature ofthe discharge tube 24 reaches 350° C. or above (time t1), the coolingfan 86 is driven to start cooling of the first position of the dischargetube 24. The temperature at the first position of the discharge tube 24starts dropping as indicated by broken line. The supply of electricpower to the heater 82 is stopped at a suitable time t2 after the timet1, the temperature in the electric furnace 80 (at positions other thanthe first position) is lowered as indicated by solid line. Thetemperature of the electric furnace 80 is lowered to the roomtemperature while the temperature at the first position is kept lowerthan the temperature at other positions of the discharge tube 24. Theheater 82 can be turned OFF at the time t2. Alternatively, the heater 82can be turned ON and OFF repeatedly after it is turned OFF at the timet2 so as to gradually lower the temperature in the electric furnace (atpositions other than the first position).

As the temperature at the first position of the discharge tube 24gradually lowers as indicated by broken line, the saturation vaporpressure of mercury at this position lowers, so that mercury isliquefied at the first position. The first position corresponds to themost cooled position shown in FIGS. 58A to 58C and liquid mercury 28 iscollected at the first position of the discharge tube 24 in the same wayas explained with reference to FIGS. 58A to 58C. As heating and coolingare conducted in this way, liquid mercury 28 is collected within a shorttime to the first position of the discharge tube 24 as the most cooledposition.

In consequence, liquid mercury can be collected within a relativelyshort time to the first position of the discharge tube 24. Thereafter,the reflector 26 and the heat conduction member 32 are fitted to thedischarge tube 24. The heat conduction member 32 is interposed betweenthe discharge tube 24 and the reflector 26 in such a fashion as to comeinto contact with the first position of the discharge tube 24 at whichliquid mercury is collected by the process described above.

FIG. 61 shows a modified example of the production apparatus of thelight source device shown in FIG. 59. Referring to FIG. 61, after thedischarge tube 24 is produced, a power source 85 inclusive of aninverter supplies power to the electrodes 25 of the discharge tube 24 toheat the discharge tube 24. The operation for heating the discharge tube24 by supplying power to the electrodes 25 of the discharge tube 24after the discharge tube 24 is produced is called “aging”. However, whenaging is merely conducted, a long time of more than hundreds of hours isnecessary to gather liquid mercury to the first position (the positioncorresponding to the heat conduction member 32).

Referring to FIG. 61, the cooling fan 86 is shown disposed in the duct88 and blows cooling air to the first position of the discharge tube 24.When aging of the discharge tube 24 is conducted while the firstposition of the discharge tube 24 is positively cooled in this way,liquid mercury can be gathered to the first position of the dischargetube 24 within a sho ter time. Aging can be conducted while thereflector 26 and the heat conduction member 32 are fitted to thedischarge tube 24. Consequently, the position of the distal end part ofthe duct 88 and the position of the heat conduction member 32 can beeasily brought into conformity with each other.

FIG. 62 shows the mercury concentration completion time when thetemperature of the first position of the discharge tube 24 is changedwhile the temperature of the discharge tube 24 is kept constant. Themercury concentration completion time represents the time necessary forcollecting almost all liquid mercury to the first position of thedischarge tube 24 (the position corresponding to the heat conductionmember 32). The temperature at portions of the discharge tube 24 (otherthan the first position) is about 80° C. due to aging. The temperatureof the first position of the discharge tube 24 that receives cooling airfrom the cooling fan 86 is lower than 80° C. The lower the temperatureat the first position of the discharge tube 24, the lower becomes thesaturation vapor pressure of the mercury and the greater becomes theliquefaction amount of mercury. Therefore, the mercury concentrationcompletion time can be shortened. The higher the temperature at portionsof the discharge tube 24 (other than the first position), the higherbecomes the saturation vapor pressure of mercury and the greater becomesthe evaporation amount of liquid mercury. The greater the amount ofgaseous mercury, the greater becomes the liquefaction amount of mercuryat the first position and the shorter becomes the mercury concentrationcompletion time.

FIG. 63 shows a modified example of the production apparatus of thelight source shown in FIG. 59. In FIG. 63, the heater 82 can heat thedischarge tube 24. Further, the power source 85 supplies electric powerto the electrodes 25 of the discharge tube 24 to heat the discharge tube24. While the heater 82 heats the discharge tube 24, aging is conducted.The cooling fan 86 is disposed in the duct 88 and blows cooling air tothe first position of the discharge tube 24. When the discharge tube 24is aged by positively heating the discharge tube 24 and positivelycooling the first position of the discharge tube 24, liquid mercury canbe collected within a shorter time to the first position of thedischarge tube 24. Aging can be conducted under the state where thereflector 26 and the heat conduction member 32 are fitted to thedischarge tube 24. Therefore, the position of the distal end of the duct88 and the position of the heat conduction member 32 can be easilybrought into conformity with each other.

FIG. 64 shows the mercury concentration completion time when thetemperature of the discharge tube 24 (portions other than the firstposition) is changed while the temperature of the first position of thedischarge tube 24 is kept constant. The temperature of the firstposition of the discharge tube 24 is set to about 60° C. by aging. Thetemperature at portions other than the first position of the dischargetube 24 readily reaches 100° C. or above. When the temperature of theportions other than the first position of the discharge tube 24 is 100°C., the mercury concentration completion time can be shortened to 40hours. When the temperature of the first position of the discharge tube24 is about 20° C. and the temperature of the portions other than thefirst position of the discharge tube 24 is 100° C., the mercuryconcentration completion time can be shortened to about 10 hours.

FIG. 65 shows a still another modified example of the productionapparatus of the light source device shown in FIG. 59. Referring to FIG.65, a mirror 90 covers the discharge tube 24. The mirror 90 reflects thelight of the aged discharge tube 24 towards the discharge tube 24.Accordingly, the discharge tube 24 is heated much more than when it ismerely aged. The cooling fan 86 is disposed in the duct 88 and blowscooling air to the first position of the discharge tube 24. In this way,liquid mercury 28 can be collected at the first position of thedischarge tube 24 in a shorter time. Incidentally, a heat insulatingmaterial may be used to cover the discharge tube 24 in place of themirror 90.

FIG. 66 shows still another modified example of the production apparatusof the light source device shown in FIG. 59. In FIG. 66, a metal plate(an aluminum plate, for example) 83 encompasses the discharge tube 24with a spacing 83A, and the heater 82 heats the discharge tube 24through the metal plate 83. The spacing 83A opens the first position ofthe discharge tube 24 to the open air and cools the discharge tube 24heated by the heater 82 at the first position. The heater 82 is arrangeddividedly in the same way as the metal plate 83, and a voltage regulator82X controls each part of the heater 82 so divided.

FIG. 67 shows a still another modified example of the productionapparatus of the light source device shown in FIG. 59. The dischargetube 24 is placed into the electric furnace 80 having the heater 80.Power is supplied to the electrodes 25 of the discharge tube 24 from thepower source 85 to conduct aging. The cooling fan 86 is disposed in theduct 88 and blows cooling air to the first position of the dischargetube 24.

FIG. 68 shows still another example of the production apparatus of thelight source shown in FIG. 59. The production apparatus of the lightsource device includes the electric furnace 80 having the heater 82 andthe cooling opening 84. A cooling metal member 94 equipped with a heatsink 94A is provided to the cooling opening 84 in such a fashion as tobe capable of moving in and out from the opening 84. A fan 95 cools thecooling metal 94 through the heat sink 94A. The cooling metal member 94keeps contact with the discharge tube 24 at the time of mercuryconcentration and cools the first position of the discharge tube 24.Since the cooling metal member 94 equipped with the heat sink 94A isused in place of the cooling fan 86 shown in FIG. 59, the range of theliquid mercury concentration portion formed at the first position of thedischarge tube 24 can be reduced. As the range of the liquid mercuryconcentration portion is made smaller, scattering and absorption oflight by the liquid mercury concentration portion during use can berestricted, and utilization efficiency of light can be improved.

FIGS. 69A to 69C show examples of the cooling metal member 94 in FIG.68. FIGS. 70A to 70C show the range 28C of the liquid mercuryconcentration portion formed at the first position of the discharge tube24 when the cooling metal member 94 shown in FIGS. 69A to 69C is used.FIG. 69A shows an example where a flat surface of the cooling metalmember 94 comes into contact with the surface of the discharge tube 24.FIG. 70A shows the range 28C of the liquid mercury concentration portionwhen the cooling metal member 94 shown in FIG. 69A is used.

FIG. 69B shows an example where an arcuate recessed surface of thecooling metal member 94 comes into contact with the surface of thedischarge tube 24. FIG. 70B shows the range 28C of the liquid mercuryconcentration portion when the cooling metal member 94 shown in FIG. 69Bis used. In FIG. 70B, the range 28C of the liquid mercury concentrationportion can be expanded in the circumferential direction of thedischarge tube 24 and can be shortened in the axial direction of thedischarge tube 24. FIG. 69C shows an example where an arcuate recessedsurface of the cooling metal member 94 keeps contact with the surface ofthe discharge tube 24 and an auxiliary metal 94B having an arcuaterecessed surface keeps contact with the surface of the discharge tube 24on the opposite side. The auxiliary metal 94B is removable. FIG. 70Cshows the range 28C of the liquid mercury concentration portion when thecooling metal 94 shown in FIG. 69C is used. In FIG. 70C, the range 28Cof the liquid mercury concentration portion can be further expanded inthe circumferential direction of the discharge tube 24 and can befurther reduced in the axial direction of the discharge tube 24.

FIGS. 71A and 71B show still another example of the production apparatusof the light source device shown in FIG. 59. In the example shown inFIGS. 71A and 71B, a rotation mechanism 96 is further provided to theproduction apparatus of the light source device shown in FIG. 59. FIG.71A is a longitudinal sectional view and FIG. 71B is a transversesectional view. The discharge tube 24 is rotated as shown in FIG. 71B.In this way, liquid mercury can be distributed in the circumferentialdirection, and the range 28C of the liquid mercury concentration portioncan be further reduced in the axial direction of the discharge tube 24as explained with reference to FIG. 70C.

FIG. 72 shows an example where the concentration position of liquidmercury and the position of the heat conduction member 32 are arrangedat the substantially center of the discharge tube 24. The electrodes 25are made of tungsten or nickel. In the conventional devices, a part ofthe mercury vapor or the rare gas is ionized during energization of thedischarge tube 24 and impinges against the electrode 25 while beingaccelerated by the electric field of the electrode 25. As a result, theelectrode 25 is sputtered and the electrode atoms spring out into thedischarge tube 24. Since the electrode atoms are activated and arestable, they adhere relatively immediately to the inner wall of thedischarge tube 24 and are stabilized. When the electrode atoms arestabilized on the inner wall of the discharge tube 24, they combine withsurrounding liquid mercury and form mercury amalgams, thereby consumingmercury. This mercury consumption amount determines the service life ofthe discharge tube 24. When most of mercury charged changes to amalgamsand can no longer contribute to light emission, mercury light emissionof the discharge tube 24 extinguishes, and light emission mainly relieson the rare gas, particularly on argon. When the emission color of thedischarge tube 24 starts changing, life of the discharge tube 24 isjudged as expired.

By arranging the light source device 18 as shown in FIG. 72, liquidmercury does not exist in the proximity of the electrodes 25. Therefore,even when the electrodes 25 are sputtered, the mercury amalgams are notformed and life of the discharge tube 24 can be extended. Theconcentration position of liquid mercury and the position of the heatconduction member 32 need not necessarily be the substantially center ofthe discharge tube 24 but may be those positions which are considerablyspaced apart from the electrodes 25. Similar effects can be obtained inthis case too.

In the discharge tube 24 in which liquid mercury is collected to thefirst position and the heat conduction member 32 is arranged at thefirst position, the mercury vapor that contributes to light emission isliquefied on the tube wall at the time of turn-off with the drop of thetemperature of the tube after turn-off. When the ambient temperature atthe time of turn-on is extremely low, however, the amount of mercury inthe proximity of the electrodes, that is first heated, is small.Consequently, the vapor pressure in the tube does not readily rise, andreddish argon light emission is sometimes observed for a long time inthe proximity of the electrodes of the discharge tube 24. Also, when auser uses a dimmer switch, turns off the discharge tube 24 under a lowbrightness state and then turns on again the discharge tube 24 (turns onthe discharge tube at a low tube current), the rise of the tubetemperature is slow, and reddish argon light emission is observed insome cases in the proximity of the electrodes of the discharge tube 24.

FIG. 73 is a graph showing the relation between the illuminating(turn-on) time of the discharge tube and chromaticity (x value) of lightemission. Curve H represents the case where a maximum tube current (14mA, for example) is applied to the discharge tube 24. Curve I representsthe case where a current of ½ of the maximum tube current is applied tothe discharge tube 24. Curve J represents the case where a current of1/10 of the maximum tube current is applied to the discharge tube 24.When a current is caused to flow through the discharge tube 24, reddishargon light emission first occurs and then mercury light emission(white) develops. In the curve H, argon light emissions lasts for about5 seconds. In the curve I, argon light emission lasts for about 30seconds. In the curve J, argon light emission lasts for about 60seconds. This is the time necessary for mercury adhering to the tubewall of the discharge tube 24 and mercury at the mercury concentrationportion to evaporate, and for gaseous mercury to move near to theelectrode 25. It can be appreciated, from this result, that the greaterthe tube current, the shorter becomes the time of argon light emission.

FIG. 74 shows the relation between the illuminating (turn-on) time ofthe discharge tube 24 and brightness. Curve K represents the relationbetween the turn-on time and brightness when the discharge tube 24 isturned on at a typical current. Curve L represents the relation betweenthe turn-on time and brightness when a maximum tube current is appliedat the initial stage of turn-on and the discharge tube 24 is then turnedon at the typical current. When the maximum tube current is applied atthe initial stage of turn-on and then (at least 5 seconds later) thedischarge tube 24 is turned on at the typical current as represented bythe curve L, the time of argon light emission can be shortened, and thetube current of brightness the user desires can be thereafter achieved.In the curve K, brightness is low at the initial stage of turn-on andbecomes gradually higher in the course of about 30 seconds. This isbecause the tube temperature gradually rises from the room temperatureand light emission efficiency of mercury rises. As represented by thecurve L, brightness is not excessively high even when the tube wall isincreased at the initial stage of turn-on. Since the elevation time ofthe tube temperature can be shortened, the dark time at the time ofinitial stage of turn-on can be shortened.

FIG. 75 shows still another modified example of the light source device.In this example, liquid mercury is collected at the first position ofthe discharge tube 24, and the heat conduction member 32 is disposed atthe first position at which liquid mercury is collected. Further, theheater 92 is disposed at a position other than the first position. Theheater 92 heats the discharge tube 24 at the initial stage of turn-on ofthe discharge tube 24. Consequently, the heater 92 promotes thetemperature elevation of the discharge tube 24 at the initial stage ofturn-on of the discharge tube 24 as well as the evaporation of mercury,and prevents argon light emission.

A light source device having high brightness can thus be obtained.Further, a backlight having high brightness and high light utilizationefficiency can be obtained, too.

FIG. 76 is a partial enlarged view of the discharge tube of theembodiment shown in FIGS. 57 to 75. Liquid mercury is collected at thefirst position of the discharge tube 24 and the heat conduction member32 is disposed at the first position at which liquid mercury iscollected (with the heat conduction member 32 being omitted in FIG. 76).The fluorescent material 30 is applied to the inner wall of the glasstube 24G of the discharge tube 24.

Particles of liquid mercury collected at the first position arerepresented by reference numeral 28P. The particles 28P of liquidmercury are a plurality of fine mercury particles. The size A (diameter)of the liquid mercury particles 28P is preferably not greater than 0.2mm. Alternatively, the liquid mercury particles 28P preferably soak intothe fluorescent material 30. The liquid mercury particle 28P positionedon the left side in FIG. 76 is put on the fluorescent material 30 whilethe liquid mercury particle 28P on the left side in FIG. 76 soaks intothe fluorescent material 30.

FIG. 77 shows a light source device similar to the one shown in FIG. 3when an impact test is conducted. Liquid mercury 28 is collected at thefirst position corresponding to the heat conduction member 32. When theimpact test is carried out, however, liquid mercury moves in some casesfrom the first position as indicated by reference numeral 28′. The lightsource device is used as a backlight of a liquid crystal display deviceand an impact of about 30 G is imparted to the backlight unit to conductthe impact test.

FIG. 78 is a graph showing the relation between the room temperature andbrightness of the discharge tube 24 before and after the impact test iscarried out, respectively. Curve M represents the relation between theroom temperature and brightness of the discharge tube 24 before theimpact test is carried out. Curve N represents the relation between theroom temperature and brightness of the discharge tube 24 after theimpact test is carried out. In the curve M, brightness of the dischargetube 24 attains maximum near the room temperature of 30° C. whereas inthe curve N, brightness of the discharge tube 24 attains maximum nearthe room temperature of 20° C. The light emission characteristics of thedischarge tube 24 are likely to change as described above. One of thecauses of the change of the light emission characteristics of thedischarge tube 24 is presumably the movement of liquid mercury from thefirst position as indicated by reference numeral 28′ in FIG. 77.

Generally, about 1 to 5 mg of mercury is charged into the discharge tube24 (2.5 mg in the embodiment). When liquid mercury 28 is gathered at thefirst position inside the discharge tube 24, liquid mercury forms one ora plurality of spherical or semi-spherical particles having a diameterof 0.3 mm or more due to surface tension. The greater the size of liquidmercury particle, the more readily it moves from the first positionbecause a particle of liquid mercury has a weight corresponding to itssize as explained with reference to FIG. 77.

In FIG. 76, the fluorescent material 30 comprises fluorescent materialparticles having diameters of several to dozens of microns, and theseparticles coarsely adhere in a thickness of 20 to 50 μm to the innerwall of the glass tube 24G of the discharge tube 24. In the fluorescentmaterial layer thus adhered, gaps in the order of dozens of microns areformed among the fluorescent material particles, and projections anddepressions of about 0.1 mm are formed on the surface of the fluorescentmaterial layer.

Therefore, when the size (diameter) A of the liquid mercury particle 28Pis not greater than 0.2 mm or when it soaks into the fluorescentmaterial 30, the liquid mercury particle 28P does not easily move evenwhen the impact is imparted to the discharge tube 24. In other words,when the liquid mercury particle 28P is as small as the size of theprojections and depressions of the surface of the fluorescent materiallayer, the contact area becomes great between the liquid mercuryparticle 28P and the fluorescent material 30, and the intermolecularforce between the mercury molecule and fluorescent material moleculefunctions as the frictional force. In consequence, even when an impactis imparted, this frictional force overcomes the impact force and theliquid mercury particle 28P does not move. When the liquid mercuryparticle 28P soaks into the gap between the fluorescent materials 30,the liquid mercury particle 28P does not move even when an impact isimparted thereto because the fluorescent material 30 functions as anobstacle. For these reasons, the light emission characteristics of thelight source device do not change even when an impact is imparted to thelight source device.

FIG. 79 is a graph showing the examination result of a movability of theliquid mercury particle 28P before the impact test is conducted andafter it is conducted at 50 G. The abscissa represents the diameter ofthe liquid mercury particle 28P and the ordinate indicates themovability. When the number of the liquid mercury particles 28P is N1before the impact test is conducted and the number of the liquid mercuryparticles 28P that do not move after the impact test is N2, themovability is defined as (N1−N2)/N1. Since the fluorescent materialexists, the number of the liquid mercury particles 28P is confirmedthrough the microscopic observation of transmission light. The number ofthe liquid mercury particles 28P can also be confirmed by X-rayobservation.

In FIG. 79, almost all the liquid mercury particles 28P having diametersgreater than 0.8 mm move by the impact of 50 G. It has been found,however, that the liquid mercury particles 28 having diameters of notgreater than 0.2 mm hardly move by the impact of 50 G. It has also beenfound through X-ray observation that the liquid mercury particles 28Psoaking into the fluorescent material 30 hardly move by the impact of 50G. Therefore, a change in the light emission characteristics of thelight source device resulting from the movement of the liquid mercuryparticles 28P does not occur.

FIG. 80 is a schematic view useful for explaining the formation of theliquid mercury particles 28P soaking into the fluorescent material 30 inthe discharge tube 24. It will be assumed in FIG. 80 that thetemperature of the inner surface of the fluorescent material 30 adheringto the inner wall of the glass tube 24G of the discharge tube 24 is T1,the temperature of the outer surface of the fluorescent material 30keeping contact with the inner wall of the glass tube G of the dischargetube 24 is T2, and the temperature of the outer surface of the glasstube G of the discharge tube 24 is T3.

The liquid mercury particle 28P existing on the left side in FIG. 80 isformed by the production apparatus (without turn-on aging) of the lightsource device shown in FIG. 59 and the liquid mercury particle 28Pexisting on the right side in FIG. 80 is formed by the productionapparatus (with turn-on aging) of the light source device shown in FIG.67.

When the first position of the discharge tube 24 is cooled withoutturn-on aging, the outer surface of the glass tube 24G of the dischargetube 24 is most cooled. Therefore, the relation, T3<T1, T2 exists.Because the thickness of the fluorescent layer is only 20 μm, there ishardly any difference between T1 and T2. Therefore, the mercury vapor ofthe glass tube 24G is sensitive to the temperatures of both T1 and T2and is liquefied with the same probability. For this reason, a seed ofliquefied mercury is formed in the proximity of the inner surface of thefluorescent material 30 as indicated by 28S1.

When the first position of the discharge tube 24 is cooled with turn-onaging, discharge referred to as a “positive column” 98 occurs in thedischarge tube 24, and this positive column 98 supplies heat and lightto the inner surface of the fluorescent material 30 of the temperatureT1. Therefore, the relations T2<T1 and T2<T3 hold, and the temperatureT2 at the contact portion between the inner surface of the glass tube24G of the discharge tube 24 and the outer surface of the fluorescentmaterial 30 becomes the lowest. Since the mercury vapor inside thedischarge tube 24 is liquefied at the lowest temperature position, aseed of liquefied mercury is formed in the proximity of the outersurface of the fluorescent material 30 as indicated by 28S2.Liquefaction of mercury spreads from the seed 28S2 of liquefied mercurytowards the inner surface of the fluorescent material 30, and theparticles 28P of liquefied mercury are collected to the first positionwhile soaking into the fluorescent material 30 in the discharge tube 24.After the liquefied mercury particles 28P soak into the fluorescentmaterial 30 in the discharge tube 24, they do not easily move even whenthe impact is subsequently imparted to them. Consequently, the lightemission characteristics of the light source device do not change.

FIG. 81 shows the backlight according to the fifth embodiment of thepresent invention. The backlight has the discharge tube 24 and the heatconduction member (cooling structure) 32 for cooling the discharge tube24. The discharge tube 24 comprises the fluorescent material 30 (notshown in FIG. 81), two electrodes 25 and mercury. Liquid mercury 28 iscollected to the center portion (the first position) of the dischargetube 24, and the heat conduction member 32 is arranged at the centerportion (the first position) of the discharge tube 24. Liquid mercury 28is vaporized to mercury vapor during use, and the fluorescent materialconverts the ultraviolet light emitted by the mercury vapor to thevisible light.

FIG. 82 shows the temperature characteristics of the conventionalbacklight and the distribution of liquid mercury. Both of thetemperature (Te) of the backlight and the amount (Hg) of liquid mercuryof the liquid crystal display device are high at opposite ends of thedischarge tube 24.

FIG. 83 shows the temperature characteristics of the backlight and theamount of liquid mercury shown in FIG. 81. The temperature (Te) of thedischarge tube 24 attains the lowest at the center portion (the firstposition) of the discharge tube 24, becomes substantially constant onboth sides of the center portion (the first position) and becomes highat both end portions of the discharge tube 24. The amount (Hg) of liquidmercury 28 is concentrated locally and mostly on the center portion (thefirst position) of the discharge tube 24.

FIG. 84 shows the temperature distribution and the amount of liquidmercury in the backlight of another example. The portion of the lowtemperature (Te) of the discharge tube 24 exists in a relatively broadregion of the discharge tube 24. The amount (Hg) of liquid mercury isnot as concentrated as in the case of FIG. 83 but is wholly concentratedthroughout the entire portion with the exception of the electrodes 25. Apart of liquid mercury 28 is evaporated at the time of turn-on butanother part of liquid mercury 28 remains as such. The portion at whichliquid mercury 28 is positioned is kept at a relatively low temperaturedue to the operation of the heat conduction member 32.

In the backlight shown in FIG. 81, the inner diameter of the glass tubeof the discharge tube 24 is D and the distance between the distal endsof two electrodes 25 is L. The existing region of liquid mercury 28 ispreferably the center region that is spaced apart by at least 20 D fromthe distal end of each electrode 25, or the center region that is spacedapart by at least 0.25L from the distal end of each electrode 25. Thefirst position is a local portion inside the region described above asshown in FIG. 83, or the whole portion inside the region described aboveas shown in FIG. 84. Incidentally, liquid mercury preferably exists inthe region spaced apart by at least 5D from the distal end of eachelectrode 25, more preferably by at least 10D, in order to avoid theinfluence of sputtered products of the electrode 25.

The diameter of liquid mercury 28 is preferably not greater than 0.2 mmas described above. To keep liquid mercury immobile at a vibration of50G, the diameter of liquid mercury 28 is preferably not greater than0.1 mm. Alternatively, the diameter of liquid mercury 28 is 0.15D orbelow, preferably 0.1D or below.

The discharge tube 24 contains a rare gas together with mercury. In oneembodiment of the present invention, the rare gas does not includeargon.

Generally, argon, as a buffer gas, and a suitable amount of mercury aresealed in the discharge tube 24. The cold cathode 25 has a sheet-like,rod-like or a cylindrical (sleeve) structure of a metal such as nickel,stainless steel or niobium. The buffer gas uses Ne—Ar. A startingvoltage becomes high when the gas pressure is high. The starting voltagebecomes low when the Ar/Ne ratio is high.

When a high voltage is applied across the electrodes 25 at both ends ofthe discharge tube 24, the electrons remaining in the discharge tube 24are attracted to the anode. While the electrons are moving at a highspeed, they impinge against argon. The cations propagated by ionizationby collision impinge against the cathode, strike out the secondaryelectrons from the cathode and trigger discharge. The electrons flowingdue to discharge impinge against the mercury atoms, and the excitedmercury radiates the ultraviolet light. The ultraviolet light excitesthe fluorescent material 30 and it emits the visible light peculiar tothe fluorescent material.

After impinging against the electrons, argon then impinges against themercury atoms and dissociates the mercury atoms. The mercury atomscontribute to discharge. The dissociation voltage of argon is 15.75 eVand the excitation voltage of argon is 11.5 eV. The dissociation voltageof mercury is 10.4 eV. Argon can be excited at 11.5 eV. Therefore, thestarting voltage becomes lower from the viewpoint of the dissociationvoltage of 15.75 eV. This is called “Penning effect”. When the ambienttemperature becomes lower, the starting voltage becomes higher becausethe mercury vapor pressure drops.

Argon is added to lower the starting voltage as described above. Whenargon is added, however, the problems of low efficiency and low lightemission quantity occur because argon invites a thermal change ofelectron energy, and absorbs the ultraviolet light emitted by mercuryand perform thermal conversion. Therefore, these problems can be solvedwhen the discharge tube 24 does not contain argon. In this case, thestarting voltage is set to a high voltage and is then adjusted to apredetermined voltage after the start.

In this embodiment, the electrode 25 comprises a carbon nano-tube. Acarbon nano-tube is a third isotope of carbon, different from diamondand graphite, and exhibits the properties of a metal or a semiconductor.The carbon nano-tube used as the electrode 25 can restrict the startingvoltage to a low level and is not easily sputtered because its meltingpoint is high. Therefore, when the carbon nano-tube is used as theelectrode 25, discharge can be easily initiated even when argon is notcontained. The amount of mercury entrapped by the sputtered productbecomes also small. The sputtered product does not adhere to the glasstube and does not invite cracking of the discharge tube due to red heatduring turn-on. Therefore, the discharge tube 24 having the electrodes25 made of the carbon nano-tube is free from degradation of performanceand can have an extended life.

It is very important to extend the life of a lamp in association withecology that will be described later. Counter-measures can be divided inaccordance with constituent materials. Development of fluorescentmaterials that can minimize degradation due to mercury and ultravioletlight is necessary. The electrode materials must have high sputteringresistance so as to reduce blackening and consumption of mercury.

Ecology presents a problem both during and after the use of thedischarge tube 24 as the cold cathode fluorescent lamp. Power loss andshort life are the problems during the use of the discharge tube 24.After the use of the discharge tube 24, the influence of the constituentmaterials on the environment are important. Recovery of mercury, inparticular, will become relatively easy if the lamp can be exchangedeasily and recovery of the device or the lamp can be completely made.(It is also important that the device construction is designed inadvance into an easily decomposable construction). Since the carbonnano-tube is made only of carbon, it is a material that is kind to theenvironment.

The heat conduction member 32 that keeps contact with the discharge tube24 and cools the first position of the discharge tube 24 is made of athermo-chromic material or a transparent material containing athermo-chromic material. In the discharge tube of the type that utilizeslight emission of mercury, efficiency and the light emission quantitydrop when the temperature excessively rises or drops. In other words,there is an optimum mercury vapor pressure depending on a mechanicalspecification. Since the mercury vapor pressure is determined by thetemperature of liquid mercury adhering to the inner wall of the tube,liquid mercury must be kept at a predetermined constant temperature inorder to always keep maximum light emission with maximum efficiencyirrespective of changes in the room temperature and the tube current.

The heat conduction member 32 made of the thermo-chromic material hasboth the function as a heat pipe for transferring heat and the functionas a heat generating body for absorbing light by the thermo-chromicmaterial and generating heat. The thermo-chromic material used hereby isa reversible material, and absorbs a greater quantity of light at atemperature lower than a predetermined temperature and a smallerquantity of light at a temperature higher than the predeterminedtemperature.

Since the temperature of the tube is low immediately after turn-on ofthe discharge tube 24, the heat pipe absorbs a greater amount of light,generates heat and rapidly heats the discharge tube 24. Therefore, theheat pipe heats liquid mercury adhering to the inside of the dischargetube 24, rapidly evaporates mercury and elevates the vapor pressure ofmercury. As the vapor pressure of mercury becomes higher, the lightemission quantity becomes all the greater, so that light absorption heatof the thermo-chromic material becomes greater and the temperaturebecomes drastically higher.

When the temperature of the thermo-chromic material exceeds thepredetermined temperature, light absorption becomes small. Consequently,only the function as the heat pipe appears, and heat is allowed toescape from the discharge tube 24 to the reflector 26 and the housingmember outside the former. Therefore, the temperature drops. In thisway, the temperature at the first position of the discharge tube 24 canbe kept at the transition temperature of the thermo-chromic material.

The thermo-chromic material is sold as “Dyna-Color Thermo-Chromic Ink”in the form of encapsulated ink from CTI Co., for example. Thisembodiment uses the ink having a transition point of 50° C. However, athermo-chromic material having a lower transition point can be used whenthe position of the thermo-chromic material is further spaced apart fromthe position of liquid mercury, and a thermo-chromic material having ahigher transition point can be used when its position is brought closerto the position of liquid mercury.

FIG. 85 shows the light source device according to the sixth embodimentof the present invention. The light source device 18 of this embodimentincludes a discharge tube 24 which contains mercury and in which liquidmercury is collected at the first position, and a cooling device forcooling the first position of the discharge tube 24. The cooling deviceincludes a cooling capacity varying mechanism for varying the coolingcapacity in accordance with the voltage (or current and voltage) appliedto the discharge tube, or the temperature at any position of the lightsource device, or the light emission quantity of the discharge tube.

In FIG. 85, the cooling device comprises a first heat conduction member32E1 keeping contact with the discharge tube 24, a second heatconduction member 32E2 keeping contact with a reflector, a movable thirdheat conduction member 32E3 capable of coming into contact with thesecond heat conduction member 32E2 and a fourth heat conduction member32E4 fixed to the first heat conduction member 32E1 and supporting thethird heat conduction member 32E3.

The first and second heat conduction members 32E1 and 32E2 each comprisea heat conduction sheet obtained by dispersing a heat conductive fillerinto a silicone resin. The third and fourth heat conduction members 32E3and 32E4 comprise a bimetal, and undergo deformation in accordance withthe change of the temperature of the discharge tube 24 to thereby bringthe third heat conduction member 32E3 into contact with the second heatconduction member 32E2 or to control the gap between the third heatconduction member 32E3 and the second heat conduction member 32E2.

A portion of the bimetal forming the fourth heat conduction member 32E4is formed into a semi-circular shape, and one of its ends is buried inand fixed to the first heat conduction member 32E1. The portion of thebimetal forming the third heat conduction member 32E3 is formed in aflat sheet-like shape. The bimetal has a sheet thickness of 0.1 mm, aradius of curvature of 2.0 mm at the semi-circular portion, and thelength of 6.0 mm at the line portion. When the temperature of the firstheat conduction members 32E1 and E2 contacting the discharge tube 24 is55° C., the third heat conduction member 32E3 comes into contact withthe second heat conduction member 32E2 as represented by broken line.

When the light source device 18 is used as the backlight of the liquidcrystal monitor, there are change of the ambient temperature (5 to 40°C., for example), and control of intensity of light of the monitor (thechange of power of the discharge tube), so the temperature of thedischarge tube 24 greatly changes. When the temperature of the dischargetube 24 changes, the brightness of the discharge tube 24 changes, too.When the current of the discharge tube 24 is lowered to 50%, thetemperature of the first heat conduction member 32E1 reaches 35° C., andthe bimetal undergoes deformation. As a result, the third heatconduction member 32E3 is spaced apart by 1 mm from the second heatconduction member 32E2 as indicated by solid line. This shape change canoffset the change of the tube temperature of 15° C. resulting from thecurrent change, and the tube temperature can be kept at a temperature ofmaximum light emission efficiency. Since the gap between the bimetal andthe heat conduction sheet can be changed substantially linearly withrespect to the tube temperature even within this temperature range, thetube temperature can be kept substantially constant.

FIG. 86 is a graph showing the relation between the spacing of thesecond heat conduction member 32E2 from the third heat conduction member32E3 and the tube temperature. This graph shows the tube temperaturewhen the spacing is changed under the same condition with a tubetemperature of 45° C. being the reference. The case where the spacingtakes a negative value indicates that the second heat conduction member32E2 and the third heat conduction member 32E3 come into mutual contactas the pressure is applied to them. The tube temperature can becontrolled at the spacing of 1 mm or more.

FIG. 87 shows a modified example of the light source apparatus shown inFIG. 85. In this example, the first heat conduction member 32Fcomprising the heat conduction sheet is so disposed as to come intocontact with, or to be spaced apart from, the discharge tube 24, and thesecond heat conduction member 32G made of the bimetal supports, at oneof its ends, the first heat conduction member 32F. The other end of thesecond heat conduction member 32G is welded to the reflector 26. Thebimetal is a Ni—Cu type bimetal having a length of 125 mm, a width of 6mm and a thickness of 0.5 mm. The first heat conduction member 32F isbonded to the surface of the bimetal on the side of a low expansioncoefficient. The first heat conduction member 32F has a length of 15 mm,a width of 6 mm and a thickness of 0.5 mm.

The bimetal is constituted so that when the ambient temperature insidethe reflector 26 is 45° C., the first heat conduction member 32F comesinto contact with the discharge tube 24. At this time, the first heatconduction member 32F is positioned so that the mid point of theeffective length of the discharge tube 24 and the center of the firstheat conduction member 32F coincide with each other within an error of 1mm. It has been found that the ambient temperature inside the reflector26 (the value at a position spaced apart by 1 mm from the reflector 26)and the surface temperature of the discharge tube 24 (with the provisothat an air layer of at least 2 mm exists around the discharge tube)satisfy the relation tabulated in Table 2 given below.

TABLE 2 Relation between Ambient Temperature and Tube SurfaceTemperature Tube Ambient Temperature 90° C. 70° C. 60° C. AmbientTemperature 55° C. 45° C. 38° C.

It can be appreciated from Table 2 that the ambient temperature and thetube surface temperature have a correlation. Assuming that the spacingbetween the discharge tube 24 and the reflector 26 is 2 mm, the meanvalue of the ambient temperature that the bimetal senses is thetemperate at the position spaced apart by 1 mm from the discharge tube24. Therefore, within the range where the tube temperature exceeds 70°C., the first heat conduction member 32F is pressed to the dischargetube 24 and heat conductivity increases. When the tube temperature islower than 60° C., the first heat conduction member 32F is spaced apartby at least 0.5 mm from the discharge tube 24, and the temperature ofthe discharge tube 24 does not drop extremely.

FIG. 88 shows a modified example of the optical apparatus shown in FIG.85. In this example, the heat conduction member 32H comprising asilicone resin molded article is disposed at a corner of the reflector24 and keeps contact with the discharge tube 24 and with the reflector24. The heat conduction member 32H has a recess 32I. A nichrome wire 32Jis disposed in the recess 32I and is fitted to the discharge tube 24 byan adhesive 32K. The nichrom wire 32J has a diameter of 0.3 mm and alength of 11 mm with the mid point of the effective length of thedischarge tube 24 being the center.

As shown in FIG. 90, one end of the nichrome wire 32J keeping contactwith the discharge tube 24 is connected to a DC power source 32L and theother end is connected to the reflector 24 kept at ground potential. Aninverter circuit 24I supplies a current of 14 mA (7W) per discharge tube24 and adjusts the intensity of light with a duty ratio. Control oflight intensity is conducted by receiving a signal voltage of 0.3 DC.The DC power source 32L supplies power to the nichrome wire 32J inaccordance with the signal voltage to the inverter circuit 24I whiletaking the relation between the signal voltage and the duty ratio of thetube current into consideration.

FIG. 89 is a graph showing the relation between the duty ratio of thetube current and feed power to the nichrome wire 32J. In this example,the heat conduction member 32H and the nichrome wire 32J togetherconstitute the cooling device. The heat conduction member 32H cools thedischarge tube 24 and the nichrome wire 32J heats the discharge tube,thereby correcting the cooling capacity of the heat conduction member32H. In this way, the heat generated by the discharge tube 24 and thenichrome wire 32J per unit length becomes substantially constant, andthe temperature of the first position of the discharge tube 24 relativeto the ambient temperature of the light source device can be keptsubstantially constant.

Further, the construction shown in FIGS. 18 to 20, the constructionshown in FIGS. 34 to 36, the construction shown in FIG. 37 and theconstructions analogous to them can be used as the cooling deviceincluding the cooling capacity varying mechanism of the light sourcedevice comprising the discharge tube which contains mercury and in whichliquid mercury is gathered to the first position, and the cooling devicefor cooling the first position of the discharge tube.

FIG. 91 shows another modified example of the optical device shown inFIG. 85. In this example, the heat conduction member 32 is fitted to thedischarge tube 24, and a Peltier device 56 is fitted to the heatconduction member 32. A black silicone sheet 100 is bonded to theposition of the reflector 26 spaced away, by about 30 mm, in thelongitudinal direction from the range in which the heat conductionmember 32 exists. This silicone sheet 100 has a size of 0.5×0.5 mm, athickness of 0.5 mm and heat conductivity of 0.15 W/K/m. A junctionportion of a thermocouple 48 is buried into the silicone sheet 100. Thetemperature of the thermocouple 48 rises as the heat absorption quantityof the silicone sheet 100 increases. In other words, the thermocouple 48attains a high temperature when large power is applied to the dischargetube 24. In this way, the light emission quantity is monitored and thePeltier device 56 is controlled.

FIG. 92 shows a display device according to still another embodiment ofthe present invention. The display device 10 includes a liquid crystalpanel 12 and a backlight 14 in the same way as the display device shownin FIG. 1. The backlight 14 includes a light guide plate 16, lightsource devices 18 disposed on either side of the light guide plate 16, ascatter reflection plate 20 disposed below the light guide plate 16 anda scatter plate 22 disposed above the light guide plate 16. Each lightsource device 18 comprises two discharge tubes 24 and a reflector 26.This light source device 18 comprises discharge tubes 24 which containmercury and in which liquid mercury is collected to the first position,and a cooling device capable of varying in cooling capacity and coolingthe first position of the discharge tube 24.

FIG. 93 is a perspective view showing the light source device 18 of thedisplay device 10 shown in FIG. 92. FIG. 94 is a transverse sectionalview of the light source device shown in FIG. 93. FIG. 95 is a side viewof the discharge tube and the cooling device as viewed from thedirection of arrow Q in FIG. 95.

Liquid mercury 28 is collected to the first position inside thedischarge tube 24. The heat conduction member 32M that is a part of thecooling device and contains an adhesive extends to two discharge tubes24 and is fitted to the first position of each discharge tube 24. One ofthe ends of a bimetal as a part of the cooling device is fitted to theheat conduction member 32M. In FIGS. 94 and 95, the other end of thebimetal 32N is shown separated from the reflector 26.

FIG. 96 is a sectional side view of the light source device 18 under thestate where the bimetal 32N extends in the same way as in FIG. 94. FIG.97 is a side view of the light source device 18 as viewed from thedirection of arrow Q. FIG. 97 is a side view of the light source device18 as viewed from arrow Q in FIG. 96. In FIGS. 96 and 97, the other endof the bimetal 32N keeps contact with the reflector 26. Therefore, thefirst position of the discharge tube 24 is cooled by the cooling devicecomprising the heat conduction member 32M and the bimetal 32N. Since thecurrent supplied to the discharge tube 24 is changed and the coolingcapacity of the cooling device is changed in this way, the brightness ofthe display screen of the display device 10 can be greatly changed.

The operation of the display device shown in FIGS. 92 to 97 will beexplained. FIG. 98 is a graph showing the relation between the tubecurrent and brightness of the conventional display device. As the tubecurrent supplied to the discharge tube 24 is increased, brightness risesbut the degree of this rise is small. It will be assumed, for example,that while the display device 10 is used at an ordinary current of 7 mA,the necessity for particularly increasing brightness of display occursand a current of 14 mA is supplied. However, even when the current isincreased from 7 mA to 14 mA, the brightness is not increased as much asexpected.

FIG. 99 is a graph showing the relation between the temperature of thefirst position (cooled portion) and brightness when liquid mercury 28 isgathered to the first position inside the discharge tube 24 and thecooling device cools the first position of the discharge tube 24. Theoperation of the light source device 18 having such features has alreadybeen explained in detail in the foregoing embodiments. In FIG. 99,maximum brightness can be acquired when the first position of thedischarge tube 24 is kept at about 70° C.

Therefore, when a current of 7 mA is supplied and the display device 10is used, the other end of the bimetal 32N separates from the reflector26 as shown in FIG. 95, so that the cooling device 24 comprising theheat conduction member 32M and the bimetal 32N does not substantiallycool the discharge tube 24. The display device 10 can be used normallyunder this condition.

Particularly when it is desired to increase brightness of display, acurrent of 14 mA is supplied. Then, the temperature of the dischargetube 24 rises. As shown in FIGS. 96 and 97, the bimetal 32N undergoesdeformation and the other end of the bimetal 32N comes into contact withthe reflector 26. The cooling device comprising the heat conductionmember 32M and the bimetal 32N cools the first position of the dischargetube 24, and the temperature of the first position of the discharge tube24 reaches about 70° C. in FIG. 99. Then, even when the temperaturerises at other portions of the discharge tube 24, the discharge tube 24emits light at maximum brightness, and brightness greatly rises.

The heat conduction member 32 uses SE4486 of Dow Corning Toray SiliconeCorporation (heat conductivity: 1.6 W/Km), for example. A compositematerial such as Ni—Cu, Cu—Zn, Ni—Mn—Fe, Ni—Cr—Fe, Ni—Mo—Fe, Ni—Mn—Cu,or the like, can be used as the high expansion material. JapaneseIndustrial Standard (JIS) classifies bimetal sheets for electricalapplications in accordance with a curve coefficient and a volumeresistivity, and this embodiment uses a kind of TM having a large curvecoefficient. The curve coefficient of the flat sheet type bimetal isexpressed by K=Dt/ΔTl². Here, K is the curve coefficient, D is adisplacement distance, t is a sheet thickness, ΔT is a temperaturedifference and 1 is an operating length.

When a bimetal having K=14×10⁻⁶/° C., t=0.5 mm and 1=20 mm is used, thedisplacement amount D is given by D=0.62 mm when the temperature risesfrom the room temperature of 20° C. to 75° C. When a current of 0.7 mAis caused to flow through the discharge tube 24 at the room temperatureof 20° C., the tube temperature rises to about 70° C. However, since thetemperature is below 75° C., the other end of the bimetal 32N remainsseparated from the reflector 26. When a current of 14 mA is caused toflow through the discharge tube 24, the tube temperature rises to 75° C.or above, and the other end of the bimetal 32N comes into contact withthe reflector 26.

FIGS. 100 to 102 represent the case where a shape memory alloy is usedfor the cooling device of the light source device of the display device.The heat conduction member 32M that is a part of the cooling device andcontains the adhesive is fitted to the first position of the dischargetube 24. One of the ends of the shape memory alloy 32O as a part of thecooling device is fitted to the heat conduction member 32M. The otherend of the shape memory alloy 32O is so arranged as to come into contactwith and out of contact from the reflector 26. FIG. 101 shows the statewhere the shape memory alloy 32O is out of contact from the reflector26. FIG. 102 shows the state where the shape memory alloy 32O keepscontact with the reflector 26.

The shape memory alloy 32O is a Ni—Ti alloy and is provided with memoryin a linear form having a diameter of 0.25 mm and a length of 30 mm. Aspring 32P pulls the shape memory alloy 32O into a curved shape. Theshape memory alloy 32O is in the martensite phase and has a smallelastic coefficient at 75° C. or below. Therefore, the shape memoryalloy 32O is out of contact from the reflector 26 as it is pulled by thespring 32P. However, when the current of 14 mA is applied to thedischarge tube 24 and its temperature reaches 75° C. or above, the shapememory alloy 32O undergoes the phase transition, the elastic coefficientbecomes great and the alloy turns to the linear shape memorized. Inconsequence” the temperature of the first position of the discharge tube24 is lowered and brightness can be efficiently improved.

FIGS. 103 to 105 represent an example where a shape memory alloy and aresin are used as the cooling device of the light source of the displaydevice. The heat conduction member 32M that is a part of the coolingdevice and contains an adhesive is fitted to the first position of thedischarge tube 24. A tube 32Q of a heat conductive silicone resinencompasses the shape memory alloy 32R as a part of the cooling device.One of the ends of this resin tube 32Q is fitted to the heat conductionmember 32M. The other end of the resin tube 32Q is so arranged as tocome into contact with, or to depart from, the reflector 26. FIG. 104shows the state where the resin tube 32Q is separated from the reflector26. FIG. 105 shows the state where the resin tube 32Q keeps contact thereflector 26. Unlike the case shown in FIGS. 100 to 102 where the shapememory alloy 32O is used alone, the contact area with the reflector 26becomes greater when the resin tube 32Q encompassing the shape memoryalloy 32R is used. Therefore, the movement of heat upon contact becomesgreat, too. The resin material is not particularly limited to siliconeso long as the tube shape changes due to the phase transition of theshape memory alloy 32O and the material has high heat conductivity.

FIGS. 106A to 106D show examples of the resin tube 32Q that encompassesthe shape memory alloy 32R. In FIG. 106A, two shape memory alloys 32Rhaving a U-shape are used. In FIG. 106B, four shape memory alloys 32Rhaving a C-ring shape are used. In FIG. 106C, one shape memory alloy 32Rhaving a U-shape is used. In FIG. 106D, a shape memory alloy 32R havinga coil shape is used.

FIG. 107 shows an example where a spring and a magnet are used as thecooling device of the light source device of the display device. Theheat conduction member 32M is fitted to the first position of thedischarge tube 24. A spring 32S having a magnetic substance 32T fittedto the tip thereof is in turn fitted to the heat conduction member 32M.An electromagnet 32U is disposed at a position opposing the magneticsubstance 32T of the reflector 26. FIG. 108 shows the state where acurrent of 7 mA is applied to the discharge tube 24 and the magneticsubstance 32T is separated from the electromagnet 32U. FIG. 109 showsthe state where a current of 14 mA is applied to the discharge tube 24and the magnetic substance 32T is attracted to, and keeps contact with,the electromagnet 32U. The width of the spring 32S and the contact areaare adjusted so that the temperature of the discharge tube 24 at thecurrent 14 mA is equal to the temperature of the discharge tube 24 atthe current 7 mA.

FIG. 110 shows an example where a ball member including a metal rod isused as the cooling device of the light source device of the displaydevice. The heat conduction member 32M is fitted to the first positionof the discharge tube 24. The ball member 32W including the metal rod Xis interposed between the heat conduction member 32M and the reflector26. The side surface of the metal rod 32X is insulated, and its end faceis a magnetic substance. A recess having an arcuate section is formed inthe surface of the heat conduction member 32M. The ball member 32W isdisposed in the recess of the heat conduction member 32M and can freelyrotate but cannot jump out from the recess of the heat conduction member32M. Further, an electromagnet 32U is disposed at a position opposingthe ball member 32W of the reflector 26.

FIG. 111 shows the state where a current of 7 mA is applied to thedischarge tube 24 and the end face of the metal rod 32X of the ballmember 32W exists at the position separated from the electromagnet 32U.At this time, the heat conduction member 32M is thermally isolated fromthe reflector 26. FIG. 112 shows the state where a current of 14 mA isapplied to the discharge tube 24 and the end face of the metal rod 32Xof the ball member 32W is attracted to, and keeps contact with, theelectromagnet 32U. At this time, the heat conduction member 32M isbrought into thermal contact with the reflector 26.

FIG. 113 shows an example where a phase transition member is used as thecooling device of the light source device of the display device. Theheat conduction member 32M is fitted to the first position of thedischarge tube 24. A container 32Y containing a plurality of capsules32Z is interposed between the heat conduction member 32M and thereflector 26. Each capsule 32Z is a glass capsule or an acrylic capsule,and a small amount of a liquid 32ZL having a boiling point of 80° C.(Fluorinate FC-84, a product of 3M Co.) is charged into the capsule.

FIG. 114 shows the state where a current of 7 mA is applied to thedischarge tube 24 and the liquid 32ZL is situated at a low positioninside each capsule 32Z because the temperature of the discharge tube 24is low. At this time, the group of the capsules 32Z has low heatconductivity, and the heat conduction member 32M is thermally isolatedfrom the reflector 26. FIG. 115 shows the state where the current 14 mAis applied to the discharge tube 24, the temperature of 80° C. istransferred to the capsules 32Z as the temperature of the discharge tube24 becomes high, hence the liquid 32ZL changes to a gas 32ZG and the gas32ZG spreads inside each capsule 32Z. At this time, the group ofcapsules 32Z has high heat conductivity (about 400 times), and the heatconduction member 32M keeps thermal contact with the reflector 26.

As described above, the display device 10 is used while a certaincurrent is applied to the discharge tube 24 in a certain case.Particularly when high brightness is desired, a higher current isapplied to the discharge tube 24 to operate the cooling device, and thedisplay device 10 can be used at maximum brightness. For example,brightness is changed over between the case where the display device isused in a dark room and the case where it is used in a bright room.Brightness is also changed over between the case where a single personuses the display device and the case where a large number of persons usethe display device. Further, brightness is changed over between the casewhere the display device is used as a display of a personal computer andthe case where it is used as a monitor. Brightness is further changedover between the case where a user watches images having low meanbrightness and the case where the user watches images having high meanbrightness. Brightness can be changed over either automatically ormanually. The user can much more enjoy dynamic images by increasing thebacklight output for the bright images and lowering the backlight outputfor the dark images. When the display device is continuously used underthe high output state, degradation of the discharge tube occurs quickly.However, life of the discharge tube can be extended when the displaydevice is used while the output is changed.

This embodiment can provide a light source device having highbrightness. Furthermore, the embodiment can provide a backlight havinghigh light utilization efficiency and high brightness.

FIG. 116 shows the liquid crystal display device including an opticalsheet and an illumination device according to the seventh embodiment ofthe present invention. In FIG. 116, the liquid crystal display device210 includes an illumination device (backlight) 212 and a liquid crystalpanel 214. The backlight 212 includes a light guide plate 216, lightsources 218 disposed on either side of the light guide plate 216, areflection plate 220 disposed below the light guide plate 216 (on thefar side from the liquid crystal panel 214) and an optical sheet 222disposed above the light guide plate 216 (on the near side of the liquidcrystal panel 214). The light source 218 comprises a discharge tube suchas a cold-cathode fluorescent tube, a hot-cathode fluorescent tube, alight emitting tube such as an EL element or an LED element, and areflector. Scattering dots 224 are formed on the lower surface of theheat guide plate 216. The light leaving from the light source 218 ismade incident to the light guide plate 216 and propagate therein whilebeing totally reflected in the light guide plate 216. A part of thelight is scattered by the scattering dots 224, is reflected by thereflecting plate 220 and leaves from the light guide plate 216. Thelight leaving from the light guide plate 216 passes through the opticalsheet 222 and is made incident to the liquid crystal panel 214.

FIGS. 117A to 117C are sectional views showing examples of the opticalsheet 222 shown in FIG. 116. FIGS. 118A to 118C are plan views of theoptical sheet 222.

In FIG. 117A, the optical sheet 222 includes a base sheet portion 228having scattering material particles 226 and a diffusion portion 230that is integrally joined to (brought into optically close contact with)the base sheet portion 228. The diffusion portion 230 has a plurality ofspaced apart projections 234 containing scattering material particles232. Valley portions 236 are defined between adjacent projections 234.The projections 234 are periodically arranged on a plane, facing to oneside. Symbol PP represents the cycle of the projections 234. Symbol HHrepresents the height of the projections 234.

In FIGS. 117B and 117C too, the optical sheet 222 includes a base sheetportion 228 including scattering material particles 226 and a diffusionportion 230 integrally joined to the base sheet portion 228. Thediffusion portion 230 has a plurality of spaced apart projections 234containing scattering material particles 232. Valley portions 236 aredefined between adjacent projections 234.

In FIG. 117A, each projection 234 has a rectangular sectional shape. InFIG. 117B, each projection 234 has a substantially rectangular sectionalshape having a rugged surface. In FIG. 117C, each projection 234 has asubstantially rectangular sectional shape having a rugged surface, andeach projection 234 comprises a group of a plurality of small scatteringmaterial particles 232 gathered together. In other words, the scatteringmaterial particles 232 are brought into close contact with one anotherthrough a binder but are not dispersed in a base material such as aresin. The projections 234 can have various shapes in this way, and thesurface of the projections 234 is not necessarily precisely stipulated.Therefore, they need not be produced so precisely unlike the prisms ofthe conventional prism sheet.

FIG. 118A shows an example of the optical sheet 222 the projections 234of which are shaped into a dot shape. FIG. 118B shows an example of theoptical sheet the projections 234 of which are shaped into a dot shape,but the depth of the valley portions 234 between each pair of adjacentprojections 234 viewed in the column direction is different from thedepth of the valley portions 235 between each pair of adjacentprojections 234 viewed in the row direction. For example, the depth ofthe valley 234 is 1 (the height of the projections 234 is 1) and thedepth of the valley portion 235 is 0.5 (the height of the projections is0.5). This arrangement can provide anisotropy to the distribution of theangles of light. FIG. 118C shows an example of the optical sheet theprojections 234 of which are formed into a long peak shape.

FIG. 119 explains the construction and operation of the optical sheet222. The optical sheet 222 is so fabricated as to possess the followingfeatures. This optical sheet 222 represents an example of a transmissiontype optical sheet. The incident light is made incident to the basesheet portion 228 of the optical sheet 222 from the light guide plate216 shown in FIG. 116. The incident light impinges against thescattering material particles 226 of the base sheet portion 228 and isscattered. The light made incident to the optical sheet 22 is scatteredlight leaving the light guide plate 216, but it contains a largeproportion of a light component having a large angle to the normal (anormal direction N) to the optical sheet 222 (curve AA in FIG. 122). Thebase sheet portion 228 converts the scattering incident light into lighthaving higher scattering property (curve BB in FIG. 122).

The light made incident from the base sheet portion 228 to the diffusionportion 230 impinges against the scattering material particles 232 ofthe projections 234 and is scattered. First, the light aa and bb leavingfrom the valley portion 236 between the two projections 234 of thediffusion portion 230 will be explained. The light aa leaves from thevalley portion 236 and travels within a predetermined range of angle αwithout contacting the adjacent projections 234. The light bb leavesfrom the valley portion 236 and is made incident to the side surface ofthe projection 234. The light bb made incident to the side surface ofthe projection 234 impinges against the scattering material particles232 therein, and on the surface of the projection 234, and is thenscattered.

In other words, the light bb made incident to the side surface of theprojection 234 is scattered inside the projection 234 or on the sidesurface of the projection 234, and outgoes as scattered light from theprojection 234. Scattered light outgoing from the projection 234contains light components in various directions. Therefore, a part ofthe ray of light outgoing from the projection 234 travels at arelatively small angle to the normal direction NN without contacting theadjacent projection 234. On the other hand, another part of the lightoutgoing from the projection 234 travels at a relatively large angle tothe normal direction NN, is made incident to other projection 234 and isfurther scattered. A part of the light scattered by other projection 234travels at a relatively small angle to the normal direction NN withoutcontacting the adjacent projections 234. Another part of the lightoutgoing from other projection 234 travels at a relatively large angleto the normal direction NN, is made incident to still another projection234 and is scattered. Therefore, among the rays of light made incidentto and scattered by a certain projection 234, the light componentdescribing a relatively large angle to the normal direction NN isconverted into the light component that gradually describes a relativelysmaller angle to the normal direction NN, and outgoing light is thusprovided gradually with directivity. Therefore, it becomes possible toobtain a broad brightness distribution such that the quantity of lightoutgoing with a relatively small angle to the normal direction NNbecomes great, and the quantity of light progressively decreases as theangle to the normal direction NN becomes greater.

Further, the light cc made directly incident from the base sheet portion228 to the projection 234 of the diffusion portion 230 impinges againstthe scattering material particle 232, is scattered and outgoes asscattered light from the projection 234. In this case too, a part of thelight outgoing from the projection 234 travels at a relatively smallangle to the normal direction NN. Another part of light outgoing fromthe projection 234 travels at a relatively large angle to the normaldirection NN is made incident to another projection 234 and is furtherscattered. Therefore, as to the light cc, the light component describinga relatively large angle to the normal direction NN among the lightoutgoing from a certain projection 234 is gradually converted into alight component describing a relatively small angle to the normaldirection NN. In consequence, outgoing light is gradually provided withdirectivity.

FIG. 121 shows a brightness distribution of the light outgoing from theoptical sheet 222. In FIG. 121, curve CC represents a brightnessdistribution of the light aa outgoing from the valley portion 236without contacting the adjacent projections 234. The predetermined rangeof angle α in FIG. 119 is set to ±45°, for example. According to thecurve CC, a bright display can be obtained when the liquid crystal panel214 is viewed from the direction of the front surface (normal directionNN).

Curve DD shows a brightness distribution of the light that outgoes fromportions near the root of the projection 234 and does not contact theadjacent projections 234. Among the light outgoing from the portionsnear the root of the projection 234, this light is the remainder of thelight that contact the adjacent projections 234 (the light describingrelatively large angle to the normal direction NN). Therefore, thislight has higher directivity to the normal direction NN than the ray oflight outgoing from the base sheet portion 228 (curve BB).

Curve EE shows a brightness distribution of light outgoing from portionsnear the distal end of the projection 234. This light has a lowprobability of contacting the adjacent projections 234 and merelyreceives the scattering operation of the scatter material particles 232.Therefore, the brightness distribution is analogous to the curve BB inFIG. 122. Curve FF represents a brightness distribution of light as thesum of the curves CC, DD and EE. This is the brightness distribution ofthe light outgoing from the optical sheet 222. The curve FF representsthe broad brightness distribution such that the quantity of lightoutgoing with relatively small angles to the front surface direction isthe greatest and the quantities of the outgoing light becomeprogressively smaller as the angles become greater from the frontsurface direction.

Also, in the present invention, scattering reflection occurs on the sidesurface of the projection 234. Therefore, the light made incident withcertain inclination, such as the light XX shown in FIG. 123, forexample, has a component of light outgoing from the optical sheet 222with inclination opposite to the inclination at the time of incidencedue to scattering reflection on the side surface of the projection 234.Therefore, the history of the incident light is eliminated.

The operation described above greatly depends on the ratio of the heightHH of the projection 234 to the pitch PP (HH/PP) and on the scatteringcapacity of the scatter material 232 of the projection 234. Therefore,the present invention sets the ratio (HH/PP) and the scattering propertyof the scatter material particles 232 to values that satisfy theoperation described above. The ratio (HH/PP) is important for settingthe predetermined range of angle a shown in FIG. 119.

Also, as represented by the curve EE in FIG. 121, the light outgoingfrom the portion of the projection 234 near the tip end thereof does notcontribute to directivity to the normal direction NN. Therefore, thequantity of light outgoing from the portion of the projection 234 nearthe tip is preferably as small as possible. For this purpose, thescattering capacity is preferably great inside and on the surface of theprojection 234 in the present invention. Scattering of the projection234 is preferably such that back scattering occurs and the probabilityof impingement of the rays of light traveling inside the projection 234against the scatter material particles 232 is high, thereby making itdifficult for the ray of light to ass through the projection 234.

FIG. 120 explains the projections 234 of the optical sheet 222 and itsvalley portions 236. In the present invention, the projection 234 maytake various shapes. In FIG. 120, the proximal portion of eachprojection and its tip and the valley portion 236 are clearlydistinguished from one another. However, the projection 234A has aprofile of a curve, and the proximal portion of the projection 234A andthe valley portion 236 cannot be clearly distinguished. Therefore, theproximal portion of the projection 234A and the valley portion 236 aredefined herein as follows. The proximal portion of the projections 234or 234A is defined as the position of HH/2 of the height HH of theprojections 234 or 234A. The valley portion 236 is defined as a portionbelow HH/2. The tip portion of the projections 234 or 234A is defined asthe position lower by 10% HH from the tip of the projections 234, 234A.

As a factor representative of scattering performance of the scatteringmaterial particles 232 of the projection 234, the ratio of the quantityof light outgoing from the tip portion of the projection 234 to thequantity of light made incident to the proximal portion of theprojection 234 is preferably 30% or below. This relation can beexpressed by S1+S2=1 and S1×T0/S2<0.3 when the area of the proximalportion of the projections 234 or 234A is S1, the quantity of light madeincident from the base sheet portion 228 to the proximal portion of theprojections 234 or 234A is T1, the area of the valley portion 236 is S2and the quantity of light made incident from the base sheet portion 228to the valley portion 236 is T0. These formulas can be re-written intoT0<0.3×(1−S1)/S1. Further, when the angle described by the center of thevalley portion 236 and two lines passing the centers of the tip of thetwo projections 234 and 234A is β, the ratio (HH/PP) is set so that theminimum value of the predetermined angle range β is not greater than±60°.

FIG. 124 shows the optical sheet 222 according to still anotherembodiment of the present invention. The optical sheet 222 shown in FIG.124 can be used as the optical sheet 222 of the backlight 212 of theliquid crystal display device 210 shown in FIG. 116. This also holdstrue of the optical sheets 222 of the following embodiments.

The optical sheet 222 comprises a base sheet portion 228 containingscattering material particles 226 and a diffusion portion 230 havingspaced apart projections 234 that are arranged periodically and containscattering material particles 232. The optical sheet 222 shown in FIG.124 is formed by the steps of forming the base sheet portion 228containing the scattering material particles 226 on a PET substrate 238and then forming the diffusion portion 230 containing the scatteringmaterial particles 232 on the base sheet portion 228 using a mesh.

The projections 234 of the diffusion portion 230 are periodicallyarranged. Each projection 234 is shaped as a whole into a rounded shapeand has a small projections and depressions structure on its surface. Inother words, the projection 234 need not have a side surface or a slopethat is accurately formed as the prisms of the prism sheet, and theoptical sheet 222 can be therefore produced relatively easily.

FIGS. 126A to 126C are explanatory views for explaining the operation ofthe optical sheet 222 shown in FIG. 124. Since the PET substrate 238 istransparent, it is omitted in FIGS. 126A to 126C. FIG. 126A shows thatthe incident ray of light is scattered by the base sheet portion 228.FIG. 126B shows that the rays of light outgo from the valley portion 236between two projections 234. FIG. 126C shows that the rays of lightoutgoing from the side surface of the projection 234 travel withoutcontacting the adjacent projection 234. The operation of the opticalsheet 222 shown in FIGS. 124 and 126A to 126C is substantially the sameas that of the optical sheet 222 shown in FIG. 119.

FIGS. 127A to 127C show the ray of light that outgoes from severalpoints of the side surface of the projection 234 and the valley portion236 without contacting the adjacent projection 234. In FIG. 124, a clearboundary does not exist between the side surface of the projection 234and the valley portion 236, and the side surface of the projection 234and the valley portion 236 continue each other. Therefore, the ray oflight outgoing from many points of the side surface of the projection234 and the valley portion 236 outgo at diversified angles.

FIG. 127A shows the ray of light outgoing from the center point of thevalley portion 236. FIG. 127B shows the ray of light outgoing from onepoint near the boundary between the valley portion 236 and the sidesurface of the projection 234. FIG. 127C shows the ray of light outgoingfrom one point near the boundary between the valley portion 236 and theside surface of the projection 234. FIG. 127D shows the ray of lightoutgoing from one point near the tip of the projection 234. FIG. 127Eshows the ray of light outgoing from the tip of the projection 234.

FIG. 125 shows a brightness gain of outgoing light of the optical sheet222 shown in FIG. 124. The brightness gain of the outgoing rays of lightof the optical sheet 222 has a broad brightness distribution such thatthe quantity of light outgoing at an angle in the front surfacedirection is maximum and progressively decreases as the angle becomesgreater from the front surface direction. However, the brightness gainat the angle in the front surface direction is about 1.5. When thisvalue is not satisfactory, the height of the projection 234 may beincreased.

Production of the optical sheet 222 shown in FIG. 124 will be furtherexplained. The base sheet portion 228 is made of an acrylic resin havinga refractive index of 1.5, and 12.5 vol % of TiO₂ beads having aparticle diameter of 1 μm and a refractive index of 2.5 are dispersed asthe scattering material particles 226 into this acrylic resin. Theacrylic resin containing the TiO₂ beads dispersed therein and an organicsolvent are mixed in a ratio of 1:3 to prepare ink. This ink is appliedto the PET substrate 238 and is dried. Since the organic solvent doesnot remain after being dried, the mixing ratio is set to be a viscositythat allows easy application. Particularly, the viscosity is adjusted sothat a flat surface is formed due to the surface tension of ink afterapplication. The layer thickness of the base sheet portion 228 is 20 μm.

The diffusion portion 230 is formed from the same ink as that of thebase sheet portion 228. In other words, an acrylic resin having arefractive index of 1.5 and containing 12.5 vol % of TiO₂ beads as thescattering material particles 232, that have a particle diameter of 1 μmand a refractive index of 2.5 and are dispersed in the acrylic resin,and an organic solvent are mixed to prepare the ink. This ink is appliedby screen-printing by using a mesh to form the diffusion portion 230having the projections 234. In this case, however, the ratio of thesolvent to the resin is 1:1 to increase the ink viscosity. Inconsequence, the mesh shape can be transferred with highreproducibility.

FIGS. 128A and 128B show a production example of the optical sheet 222by screen-printing using the mesh. FIG. 129 shows an example of the meshused in FIGS. 128A and 128B. The mesh (silk screen for silk printing)240 comprises polyester filaments 240A and 240B that are entangled withone another in both transverse and longitudinal directions into a wovenfabric shape. This example uses a 120 meshes/inch polyester mesh (wirediameter: 46 μm, opening: 149 μm square, thickness: 80 μm). The gapbetween two polyester filaments 240A and 240B is greater than the wirediameter of the polyester filaments 240A and 240B.

As shown in FIG. 128A, the ink 242 described above is applied to the PETsubstrate 238 and is then dried to form the base sheet portion 228. Themesh 240 is then put on the base sheet portion 228 and the ink 242 isapplied. As shown in FIG. 128B, the mesh 240 is removed to form thediffusion portion 230. In this case, the viscosity of the ink isincreased so that the mesh shape can be transferred with highreproducibility as described above.

The projections 234 formed thereby have a size of 149 μm square, aheight of 80 μm and a pitch of 195 μm. (Note, the projections arecondensed during the drying and solidification process and the shapebecomes blunt due to the surface tension of the adhesive). Since thediffusion portion 230 is made of the same material as the base sheetportion 228, they are optically integrated with each other (withoutforming the interface of the refractive index).

FIGS. 130A to 130C show another production example of the optical sheet22 by using the mesh. As shown in FIG. 130A, the ink is applied to thePET substrate 238 and is dried to form the base sheet portion 228. Theink 242 is then applied. As shown in FIG. 130B, the mesh 240 is pushedto the ink 242. The mesh 240 is thereafter removed to form the diffusionportion 230 as shown in FIG. 130C.

FIGS. 131A to 131C show another production example of the optical sheet222 by use of the mesh. As shown in FIG. 131A, the mesh 240 comprisesfilaments 240A and 240B that are entangled with each other into a wovenfabric shape in both longitudinal and transverse directions. The wirediameter of the filament 240A is different from that of the filament240B. FIG. 131B is a sectional view of the optical sheet 222 produced byusing the mesh 240 shown in FIG. 131A when taken along a line XVIB-XVIBin FIG. 131A. FIG. 131C is a sectional view of the optical sheet 222produced by using the method 240 shown in FIG. 131A when taken along aline XVIC-XVIC in FIG. 131A.

FIG. 132 is a graph showing a brightness distribution of the opticalsheet 222 produced by using the mesh 240 shown in FIGS. 131A to 131C.Curve FF represents brightness when viewed in the section shown in FIG.131B, and curve GG represents brightness when viewed in the sectionshown in FIG. 131C.

FIG. 133 shows another production example of the optical sheet 222 byusing the mesh. A roller 240R equipped with a mesh pattern is caused totravel on the base sheet portion 228 to form the projections 234.Reference numeral 244 denotes an ink applicator.

FIG. 134 shows an example of a mask 246 when the mask is used to producethe optical sheet 222. In the mask 246, holes 246 a are formed in ametal sheet by etching, for example. The holes 246 a are arranged in asquare arrangement. This mask 246 is used in place of the mesh 240 shownin FIGS. 128A, 128B and 130A to 130C to produce the optical sheet 222 asshown in these drawings.

FIG. 135 shows another example of the mask 246 when the mask 246 is usedto produce the optical sheet 222. In the mask 246, holes 246 a areformed in a metal sheet by etching, for example. The holes 246 a areperiodically arranged in the columnar direction but are arranged zigzagin the longitudinal direction. This mask 246 is used to produce theoptical sheet 222 in the same way as in FIG. 134.

FIG. 136 shows another example of the optical sheet 222 produced by useof the mask shown in FIG. 134 or 135. When the mesh 240 is used, acommercially available mesh 240 may be used. However, since the heightof the projections 234 depends on the diameters of the filaments 240Aand 240B, high projections 234 cannot be produced so easily. When themask 246 is used, however, high projections 234 can be formed.

A production example of the optical sheet 222 using the mask 246 will beexplained. To fabricate the base sheet portion 228 and the diffusionportion 130, an acrylic resin having a refractive index of 1.5 andcontaining 12.5 vol % of TiO₂ beads having a refractive index of 2.5 anda particle diameter of 2 μm, and dispersed in the resin as the scattermaterials 226 and 232, and an organic solvent, are mixed first toprepare ink. The thickness of the mask 246 is 185 μm. The size of theholes 46 a is 280 μm. The pitch of the holes 246 a is 350 μm. In thiscase, the holes 246 a are arranged in a square arrangement as shown inFIG. 134. A 50 μm-thick base sheet portion 228 is formed on a PEPsubstrate 238, and the diffusion portion 230 is formed on the base sheetportion 228 by use of the mask 246. The beads are larger than the beadsof the example shown in FIG. 124, and the pitch of the holes 246 (pitchof the projections 234) is changed to match the larger beads.

Immediately after printing, the projections 234 have a size of 280 μmsquare, a height of 185 μm and a pitch of 350 μm. After drying, theprojections 234 have a size of 221 μm square, a height of 145 μm and apitch of 350 μm. (The projections are condensed during the drying andsolidification process and the shape becomes dull due to the surfacetension of the adhesive). In this way, high projections 234 can beformed and a brightness gain of 2.1 can be accomplished.

Next, a production example of the optical sheet 222 having higherprojections 234 will be explained. Ink for forming the base sheetportion 228 and the diffusion portion 230 is the same as that of theforegoing embodiments. The mask 246 has a thickness of 800 μm, a holesize of 280 μm square and a hole pitch of 350 μm. In this case, theholes 246 a are arranged zigzag as shown in FIG. 135. After printing anddrying, the projections 234 have a size of 221 μm, a height of 600 μmand a pitch of 350 μm. In this way, the projections 234 having a greaterheight can be formed, and a brightness gain of 2.5 can be achieved.Since the projections 234 are arranged zigzag, uniformity of the pitchdrops. Therefore, this optical sheet is combined with a device having auniform pitch such as a liquid crystal panel to reduce the possibilityof occurrence of moiré.

FIGS. 137A to 137C shows a production example of the optical sheet byuse of a metal roll. In FIG. 137A, the base portion 228 is applied anddried onto the PEP substrate 238. The sheet 245 having the ink 242formed thereon is passed between a metal roll 248 and a roll 249. FIG.137B shows a part of the metal roll 248 and FIG. 137C shows thediffusion portion 230 having the projections 234 formed of the ink 242.

FIG. 138 shows an application example of the optical sheet 222 of FIG.124. In FIG. 138, the optical sheet 250 comprises a first optical sheet222 and a second optical sheet 252 laminated on each other. The firstoptical sheet 222 corresponds to the optical sheet 222 shown in FIG.124. The second optical sheet 252 comprises a transparent PET resinlayer 252 a and a diffusion portion 252 b having projections 234 acontaining a scattering material. The diffusion portion 252 b has astructure similar to that of the diffusion portion 230 of the opticalsheet 222 shown in FIG. 124.

The first optical sheet 222 accomplishes a brightness distributionhaving high directivity. Therefore, the second optical sheet 252 neednot have the scattering layer corresponding to the base sheet portion228 of the first optical sheet 222, but may well have a transparentresin layer 252 a. However, a thin scattering layer, or a scatteringlayer having a small difference between the refractive index of thescattering material and the refractive index of the base resin, or asmall amount of the scattering layer may be used in place of thetransparent resin layer 252 a.

When the brightness gain of the optical sheet 222 shown in FIG. 124 islower than a desired value, the desired brightness gain (for example, 2)can be accomplished when the first optical sheet 222 and the secondoptical sheet 252 are laminated as in the case of the optical sheet 250shown in FIG. 138. In this case, in the liquid crystal display device210 shown in FIG. 116, the optical sheet 250 is used in place of theoptical sheet 222 shown in FIG. 116.

FIG. 139 shows another application example of the optical sheet 22 ofFIG. 124. In the optical sheet 222 shown in FIG. 124, the base sheetportion 228 is formed on the PET substrate 238. In FIG. 139, the basesheet portion 228 is directly formed on the light guide plate 216. Theoperation of the optical sheet 222 in this case is the same as that ofthe foregoing embodiment. This optical sheet 222 is produced in thefollowing way. To form the base sheet portion 228 and the diffusionportion 230, ink is first prepared by mixing an acrylic resin having arefractive index of 1.5 and dispersing therein 42.2 vol % of TiO₂ beadshaving a particle diameter of 2 μm and a refractive index of 2.5 as thescatter materials 226 and 232, and an organic solvent.

The mask 262 has the thickness of 185 μm, the hole size of 280 μm squareand the hole pitch of 350 μm for the holes 246 a. In this case, theholes 246 a are arranged zigzag as shown in FIG. 135. The base sheetportion 228 is formed on the light guide plate 216 to the thickness of20 μm, and the diffusion portion 230 is formed on the base sheet portion228, using the mask 246. The resulting projections 234 have the size of221 μm square, the height of 145 μm and the pitch of 350 μm. Scatteringdots 224 shown in FIG. 116 are then formed on the lower surface of thelight guide plate 216. The optical sheet 222 having excellentdirectivity can be accomplished in this way. Since this optical sheet222 is integrated with the light guide plate 216, another component neednot be added, and the production cost can be lowered.

The optical sheet explained above constitutes the liquid crystal displaydevice in cooperation with the light source, the light conduction plateand the liquid crystal panel. Similarly, the following optical sheetconstitutes the liquid crystal display device.

FIG. 140 shows another application example of the optical sheet 222 ofFIG. 124. This example represents an example of direct illumination typebacklight 254. The direct illumination type backlight 254 is disposedjust below the liquid crystal panel 214 and includes a plurality oflamps 256 and a reflector 258 arranged in one plane. The optical sheet222 is disposed between a plurality of lamps 256 and the liquid crystalpanel 214. In the direct illumination type backlight 254, thedistribution of light is different between a position II near the lamp256 and a position JJ far from the lamp 256. Therefore, when such a rayof light is made incident to the liquid crystal panel 214, unevenbrightness occurs in the liquid crystal panel 214. The optical sheet 222includes the base sheet portion 228 and the diffusion portion 230 asdescribed above, and there is a function so that the incident ray oflight in an inclined direction is scattered and caused to be directed tothe normal direction. Consequently, the ray of light having an excellentdistribution with the front surface direction as the center is madeincident to the liquid crystal panel 214, and an uneven brightness doesnot occur in the liquid crystal panel 214.

As described above, the ray of light is scattered more vigorously by theprojections 234 when the scattering performance of the scatteringmaterial particles 232 is greater, and directivity becomes higher. Inthe foregoing embodiment, the scattering performance of the scatteringmaterial particles 232 of the projections 234 is expressed by the ratioof the quantity of the ray of light outgoing from the tip portion of theprojection 234 to the quantity of the ray of light made incident to theproximal portion of the projection 234. Here, the scattering performanceof the scatter material 232 of the projections 234 is expressed by thedifference of refractive indices (Δn) between the refractive index (n0)of the resin constituting the diffusion portion 230 and the refractiveindex (n1) of the scattering material particles 232 (where Δn is anabsolute value of (n0-n1)), and the density of the scattering materialparticles 232 (or volume percent) in the resin constituting thediffusion portion 230. When the size of the scattering materialparticles 232 becomes 1 μm or below, interference and the Mie scatteringeffect start appearing. In consequence, an undesired scatteringcondition is established, and the size of the scattering materialparticles 232 is preferably at least 1 μm.

Table 3 given below tabulates the simulation result of determining thebrightness gain of the transmission type optical sheet 222 by using theratio (H/P) of the height of the projection 234 to the pitch, therefractive index difference Δn and the density of the scatteringmaterial particles 232 as the parameters. The result that provides thebrightness gain of 1.5 or more is judged as good. Incidentally, anexample of Δn=1 corresponds to the case where the scattering materialparticles 232 uses polycarbonate, and Δn=0.05 corresponds to the casewhere the scattering material particles 232 uses silica. As to thedensity of the scattering material particles 232, the case where thediameter of the scattering material particles 232 is 0.5% P is used asthe reference. In other words, when the diameter of the scatteringmaterial particles 232 is 2 μm and the pitch P is 400 μm, the density is50 pcs/pitch. Similarly, when the diameter of the scattering materialparticles 232 is 1 μm and the pitch P is 200 μm, the density is 50pcs/pitch. Further, 50 pcs/pitch corresponds to 1.56 vol %, 10 pcs/pitchcorresponds to 12.5 vol %, and 100 pcs/pitch corresponds to 12.5 vol %.

TABLE 3 Δn 0.05 0.1 0.5 1.0 H/P = 1.5  50 (N/P) 100 (N/P) 1.6 150 (N/P)1.9 3.3 H/P = 1.0  50 (N/P) 1.8-1.9 100 (N/P) 1.4 1.5 2.3 2.3-2.5 150(N/P) 1.6 H/P = 0.4  50 (N/P) 1.5-1.7 100 (N/P) 1.7-2.0 150 (N/P) 2.1H/P = 0.3  50 (N/P) 100 (N/P) 1.5 150 (N/P) 2.0

(N/P) is the number N of the scattering material particle 232 per thepitch PP.

It can be understood from Table 3 that the ratio (H/P) of the height HHof the projections 234 to the pitch PP is preferably at least 0.3, therefractive index difference Δn (where Δn is an absolute value of(n0-n1)) between the index (n0) of the base material of the diffusionportion 230 and the refractive index (n1) of the scattering materialparticles 232 is preferably 0.05, and the density of the scattermaterial 232 in the diffusion portion 230 is preferably at least 50(N/P). When the size of the scattering material particles 232 changes,the values of the respective factors can be selected so as to acquireequivalent scattering performance.

Table 4 given below tabulates the simulation result of determining thebrightness gain of the transmission type optical sheet 222 by using theratio (H/P) of the height of the projection 234 to the pitch, therefractive index difference Δn and the density of the scatteringmaterial particles 232 as the parameters when the optical sheet 222 isintegrated with the light conduction plate 216 as shown in FIG. 139.

TABLE 4 Δn 0.05 0.1 0.5 1.0 H/P = 1.5  50 (N/P) 100 (N/P) 150 (N/P) 23.0 H/P = 1.0  50 (N/P) x x 1.4 1.2-1.3 100 (N/P) x x 1.4 1.5-1.8 150(N/P) 1.9-2.5 H/P = 0.4  50 (N/P) x x 1.1 x 100 (N/P) x x 1.1 1.1-1.2150 (N/P) 2.0

It can be understood from Table 4 that the ratio (H/P) of the height HHof the projections 234 to the pitch PP is preferably at least 1.0, therefractive index difference Δn is preferably 0.05, and the density ofthe scattering material 232 in the diffusion portion 230 is preferablyat least 100 (N/P). When the size of the scattering material 232changes, the values of the respective factors can be selected so as toacquire the equivalent scattering performance.

FIG. 141 shows an example where the optical sheet 222 is used as areflection type optical sheet. In this case, a reflecting mirror 260 isdisposed on the base sheet portion 228. The base sheet portion 228 andthe diffusion portion 230 are made of an acrylic resin having arefractive index of 1.5. As the scattering material particles 226, 42.4vol % of SiO₂ beads having a particle diameter of 2 μm and a refractiveindex of 1.55 are dispersed in this acrylic resin. The acrylic resinhaving the SiO₂ beads dispersed therein and an organic solvent are mixedto give ink. The refractive index difference Δn is 0.05.

This ink is applied to the PET substrate 238 to a thickness of 200 μm,and the projections 234 are then formed by using the metal roll 248shown in FIG. 137A. The metal roll 248 has a plurality of blades havinga pitch of 200 μm and a depth of 350 μm. While being rotated, the bladesare urged to the ink, thereby cutting the ink having a thickness of 300μm into the base sheet portion 228 having a thickness of 100 μm and thediffusion portion 230 having a thickness of 350 μm. After this ink isfurther dried, the reflecting mirror 260 is disposed on the surface ofthe PET substrate 238 on the opposite side.

In the reflection type optical sheet 222, the light is made incidentfrom the diffusion portion 230 to the optical sheet 222, passes throughthe diffusion portion 230 and the base sheet portion 228, is reflectedby the reflecting mirror 260, passes again through the base sheetportion 228 and the diffusion portion 230, and outgoes from thediffusion portion 230. The light finally passes through the base sheetportion 228 and the diffusion portion 230, and outgoes from thediffusion portion 230. Therefore, the operation of the reflection typeoptical sheet 222 is similar to that of the transmission type opticalsheet 222 described above.

In the reflection type optical sheet 222, a lot of light is madeincident to the projections 234 at many angles. In the reflection typeoptical sheet 222, the light is made incident to the projections 234 andmust pass through the projections 234 and the base sheet portion 228 toreach the reflecting mirror 260 while being scattered. Therefore, itcannot be said that the scattering performance of the projections 234 ispreferably as high as possible. It is therefore necessary for theprojections 234 to have a low scattering performance so that the ray oflight can reach the reflecting mirror 260, and a sufficient scatteringperformance so that the ray of light reflected by the reflecting mirror260 can be scattered and exhibit directivity described above. For thispurpose, it is preferred to use beads having a lower scattering capacityso as to allow easy passage of the ray of light and to allow thereflected ray of light to travel in a long distance to provide thedesired scattering effect. For this reason, the reflection type opticalsheet 222 preferably has higher projections 234 than the transmissiontype optical sheet 222. Incidentally, the reflecting mirror 260 may beeither a mirror surface reflecting mirror or a scattering-reflectingmirror. The reflecting mirror may be produced by vacuum evaporating Al,Ag, or the like, to the PET substrate 238. The base sheet portion 228may also be formed on the reflecting mirror (sheet) 260 by omitting thePET substrate 238.

Table 5 given below tabulates the simulation result of determining thebrightness gain of the reflection type optical sheet 222 by using theratio (H/P) of the height HH of the projection 234 to the pitch PP, therefractive index difference Δn and the density of the scatteringmaterial particles 232 as the parameters.

TABLE 5 Δn 0.05 0.1 1.0 H/P = 11.5  50 (N/P) 2.5 2 x 100 (N/P) 4.1 3.6 x150 (N/P) 9 7.4 H/P = 5.7  50 (N/P) 2 1.8 x 100 (N/P) 2.5 2.3 x 150(N/P) 5 4.1 H/P = 2.8  50 (N/P) 2 1.8 x 100 (N/P) 2 2 x 150 (N/P) 3 2.5Δn 0.05 0.1 0.1 H/P = 1.9  50 (N/P) 100 (N/P) 150 (N/P) 2.2 Δn 0.05 0.11.0 H/P = 1.1  50 (N/P) 100 (N/P) 150 (N/P) 1.7

It can be appreciated from Table 5 that the ratio (H/P) of the height HHof the projections 234 to the pitch PP is preferably at least 1.1, therefractive index difference Δn (Δn is an absolute value of (n0-n1)) ispreferably 0.1 or below, and the density of the scattering materialparticles 232 in the diffusion portion 230 is preferably at least 50(N/P). When the size of the scattering material particles 232 changes,the values of the respective factors can be selected so as to acquireequivalent scattering performance.

FIG. 142 shows an example of a liquid crystal display device wherein thereflection type optical sheet is disposed below the light guide plate.The light guide plate 216 is disposed below the liquid crystal panel214. A light source 218 comprising a lamp and a reflector is disposed oneach side of the light guide plate 216. The reflection type opticalsheet 222 is disposed below the light guide plate 216 (on the sideopposite to the liquid crystal panel 214). Scattering dots 224 areformed on the lower surface of the light guide plate 216. A scatteringray of light is made incident from the light guide plate 216 to thereflection type optical sheet 222 through the scattering dots 224, andthe directive light reflected by the reflection type optical sheet 222passes through the light guide plate 216 and is made incident to theliquid crystal panel 214. The optical sheet need not be interposedbetween the light guide plate 216 and the liquid crystal panel 214 ashas been required in the prior art. However, a scattering sheet or atransmission type optical sheet may further be interposed between thelight guide plate 216 and the liquid crystal panel 214.

FIGS. 143A and 143B show another modified example of the liquid crystaldisplay device of FIG. 142. This example uses a light guide plate 216having projections 224 a formed thereon in place of the light guideplate 216 having the scattering dots of FIG. 142. The projections 224 aare formed simultaneously with the formation of the light guide plate216. FIG. 143B is an enlarged view of one projection 224 a shown in FIG.143B. The projection 224 a has side surfaces 224 b which are inclined tothe lower surface of the light guide plate 216. When the angle ofinclination of the side surface 224 b is appropriately set, a part ofthe ray of light totally reflected inside the light guide plate 216 isallowed to outgo towards the reflection type optical sheet 222.

The angle of inclination of the side surface 224 b is preferably 10° orbelow, more preferably 5° or below. When the angle of inclination of theside surface 224 b is set in this way, the light is efficiently allowedto outgo from the light guide plate 216 towards the reflection typeoptical sheet 222 through the projections 224 a. In the case of thelight conduction plate 216 having the scattering dots 224 of FIG. 142,on the other hand, light components that scatter rearward develop in thescattering dots 224. The light components scattering rearward outgo fromthe light guide plate 216 at an angle outside a predetermined anglerange, and a loss occurs.

FIGS. 144A and 144B show a modified example of the light guide plate 216of FIGS. 143A and 143B. FIG. 144A shows a light guide plate 216 theprojections 224 a of which have different height. FIG. 144B shows thelight guide plate 216 the pitch of the projections 224 a of which isdifferent. In this way, the shape and the pitch of the projections 224 aof the light guide plate 216 as outgoing means to the reflection typeoptical sheet 222 need not always have a predetermined shape and apredetermined pitch. When the shape and/or the pitch of the projections224 a is changed, light direction characteristics can be changed.

FIG. 145 shows another application example of the reflection typeoptical sheet 222 of FIG. 41. This example represents an example of thedirect illumination type backlight 262. The direct illumination typebacklight 254 is disposed just below the liquid crystal panel 214 andincludes a plurality of lamps 256 and reflectors 258 arranged in oneplane. The optical sheet 222 is disposed on the opposite side of theliquid crystal panel 214 to the lamps 256. In the direct illuminationtype backlight 254, the distribution of light is different between theposition near the lamp 256 and the position far from the lamp 256.Therefore, when such ray of light is made incident to the liquid crystalpanel 214, uneven brightness occurs in the liquid crystal panel 214. Theoptical sheet 222 includes the base sheet portion 228 and the diffusionportion 230 as described above. Therefore, the ray of light havingdirectivity is made incident to the liquid crystal panel 214, and unevenbrightness of the liquid crystal panel 214 does not occur.

FIG. 146 shows another application example of the reflection typeoptical sheet 222 of FIG. 141. In this example, the reflection typeoptical sheet 222 is brought into optically close contact with the lightguide plate 216 through a transparent resin layer 264. The transparentresin layer 264 is made of the same acrylic resin having a refractiveindex of 1.5 as the base material of the reflection type optical sheet222. The light guide plate 216 has a silver reflection film 266 havingdifferent areas that are greater in the proximity of the light source218 and become progressively smaller towards the center. Therefore, theray of light reflected by the reflection film 266 among thosepropagating in the light guide plate 216 continues to travel in thelight guide plate 216, and the ray of light made incident to thetransparent resin layer 264 through the gaps in the reflection film 266are made incident to the reflection type optical sheet 222. In thisexample, the reflection film 266 controls the transmission quantity oflight so that the proportion of the incident ray of light is decreasednear the light source where the quantity of light is great in the lightguide plate 216 but is increased at the center portion where thequantity of light in the light guide plate 216 is great. The light socontrolled is made incident to the reflection type optical sheet 222 andreceives the action of this sheet 222 in the same way as describedabove. Since the reflection type optical sheet 222 is integrated withthe light guide plate 216, the number of assembly steps can be reducedand the rate of defective products can be reduced.

FIG. 147 shows still another application example of the reflection typeoptical sheet 222 of FIG. 144. In this example, a transparent resinlayer 265 of the same material as the acrylic resin of the base materialof the reflection type optical sheet 222 is applied to (brought intooptically close contact with) the projections 234 of the optical sheet222, and is cured under a flat state. A transparent resin 268 (opticaladhesive) bonds the transparent resin layer 265 to the light guide plate216. The adhesive 268 is formed as dots the area of which is small inthe proximity of the light source 218 and becomes progressively greatertowards the center. The ray of light the incidence quantity of which isadjusted is made incident to the reflection type optical sheet 222. Theray of light receive the similar action of the reflection type opticalsheet 222 described above.

FIGS. 148A to 148D show further examples of the optical sheet. FIG. 148Ashows the optical sheet 222 similar to the optical sheet 222 shown inFIGS. 117A to 117C. In other words, this optical sheet 22 includes thebase sheet portion 228 containing the scattering material particles 226and the diffusion portion 230 integrally joined to the base sheetportion 228. The diffusion portion 230 has a plurality of projections234 containing the scattering material particles 232 and the valleyportions 236 formed between the projections 234.

In FIG. 148B, the optical sheet 222 includes the transparent base sheetportion 228A and the diffusion portion 230 integrally bonded to the basesheet portion 228A. The diffusion portion 230 has a plurality ofprojections 234 containing the scattering material particles 232 and thevalley portions 236 formed between the projections 234. When the lightmade incident to the optical sheet 222 is scatted light having higherscattering performance than those represented by the curve AA in FIG.122, for example, a transparent base sheet portion 228A may be used inplace of the base sheet portion 228 containing the scattering materialparticles 226.

In the present invention, the construction of the diffusion portion 230is important to improve directivity of light, and the base sheet portion228 containing the scattering material particles 226 need not always bejoined integrally with the diffusion portion 230. When the scatteringoperation of the base sheet portion 228 containing the scatteringmaterial particles 226 is necessary, a scattering sheet formedseparately may be laminated with the diffusion portion 230 withoutintegrally joining the base sheet portion 228 to the diffusion portion230.

In FIG. 148C, the optical sheet 22 includes the diffusion portion 230having a plurality of spaced apart projections 234 containing thescattering material particles 232, and the valley portions 236 formedbetween projections 234. In other words, this optical sheet 222 does notcontain the base sheet portion 228 shown in FIG. 148A or the base sheetportion 228A shown in FIG. 148B. The diffusion portion 230 may besupported by any other support structures 290.

Referring to FIG. 148D, the optical sheet 222 includes the transparentbase sheet portion 228A and the diffusion portion 230 integrally bondedto the base sheet portion 228A. The diffusion portion 230 has aplurality of projections 234 containing the scattering materialparticles 232 and the valley portions 236 formed between the projections234. The projections 234 comprise a group of small scattering materialparticles 232 gathering in the same way as in FIG. 117C. In other words,the scattering material particles 232 are brought into close contactwith one another by a binder, but are not dispersed in the base materialsuch as the resin.

FIGS. 149A to 149E show still further example of the optical sheet ofFIG. 148A. The optical sheet 222 shown in FIG. 149A is fabricated byfurther disposing a transparent material layer 292 on one of the sidesof the diffusion portion 230 in such a fashion as to substantially fillthe valley portions 236. In other words, the optical sheet 222A includesthe base sheet portion 228 containing the scattering material particles226, the diffusion portion 230 integrally joined to the base sheetportion 228 and having a plurality of projections 234 containing thescattering material particles 232 and the valley portions 236 formedbetween projections 234, and the transparent material layer 292substantially filling the valley portions 236. In FIG. 149A, thetransparent material layer 292 is shown formed to the same height as thetip of the projection 234. However, it is also possible to form thetransparent material layer 292 to the thickness higher than the tip ofthe projections 234 so that the transparent material layer 292completely covers the tips of the projections 234. The transparentmaterial layer 292 is analogous to the transparent resin layers 264 and265 shown in FIGS. 146 and 147. As the transparent material layer 292 isdisposed, it buries the projections 234, and the optical sheet 22Ahaving flat upper and lower surfaces can be obtained.

The projections are formed by scattering the scattering materialparticles 232 in the resin as the base material, and the transparentmaterial layer 292 is made of the resin. Here, when the refractive indexof the base material of the projections 234 is (n0) and that of thetransparent material layer 292 is (n2), the materials can be selected tosatisfy the relation n0=n2. In this way, total reflection does not occuron the interface between the projections 234 and the transparentmaterial layer 292, and the scattered light is allowed to more easilyleave from the projections 234 to the transparent material layer 292.Further, the materials can be selected so as to satisfy the relationn0<n2. In this way, probability of total reflection, on the surface ofthe projections 234, of the ray of light describing relatively smallangles to the normal direction N among those traveling from thetransparent material layer 292 to the projections 234 increases, anddirectivity becomes higher.

The optical sheet 222A shown in FIG. 149B is the same as the opticalsheet 222A shown in FIG. 149A with the exception that the base sheetportion 228 containing the scattering material particles 226 is changedto the transparent base sheet portion 228A. The operation of thisoptical sheet 222A is the same as that of the optical sheet 222A shownin FIG. 149A.

The optical sheet 222A shown in FIG. 149C has the diffusion portion 230having a plurality of spaced apart projections 234 containing thescattering material particles 232 and the valley portions 236 formedbetween the projections 234, and the transparent material layer 292substantially filling the valley portions 236. In this optical sheet222A, the transparent material layer 292 serves as the support structureof the projections 234. In this optical sheet 222A, therefore, the basesheet portion 228 or 228A can be omitted in the same way as in FIG.148C. The operation of this optical sheet 222A is the same as that ofthe optical sheet 222A shown in FIG. 149A.

The optical sheet 222A shown in FIG. 149D is the same as the opticalsheet 222A shown in FIG. 149C with the exception that the projection 234comprises a group of a plurality of small scattering material particle232 gathering together.

The optical sheet 222A shown in FIG. 149E is the same as the opticalsheet 222A shown in FIG. 149C or 149D with the exception that theprojections 234 have the inclined side surfaces. When this constructionis employed, the angles of the ray of light totally reflected on thesurfaces of the projections 234 become smaller whenever they are totallyreflected as indicated by arrow G when the materials are so selected asto satisfy the relation n0<n2. In consequence, directivity becomeshigher.

FIGS. 150A to 150E show still further examples of the optical sheet. Theoptical sheet 222B shown in FIG. 150A includes the diffusion portion 230having a plurality of spaced apart projections 234 facing to one sideand the valley portions 236 formed between the projections 234. A layer294 having scattering property is disposed on the surface of theprojection 234B. The protuberances 234B are integrally disposed with thebase sheet portion 228B. The projections 234B in this example do notcontain any scattering material particles such as the scatteringmaterial particles 232 of the projections 234 explained so far. However,the layer 294 having the scattering property and disposed on theprojections 234B operate in the same way as the projections 234containing the scattering material particles 232 of the foregoingembodiments.

In the optical sheet 222B shown in FIG. 150B, the layer 294 having thescattering property covers the entire surface of the projections 234Band the valley portions 236. The operation of this optical sheet 222B isthe same as the operation of the optical sheet 222B shown in FIG. 150A.The optical sheets 222B shown in FIGS. 150A and 150B can be produced bythe steps of molding a resin containing no scatter material to obtainthe diffusion portion 230 having the projections 234B and the valleyportions 236, and applying the layer 294 having the scattering propertyonto the projections 234B or bonding a scatter sheet. Therefore, theproduction is easy.

The optical sheet 222B shown in FIG. 150C is formed by adding atransparent material layer 292 for filling the valley portions 236 ofthe optical sheet 222B shown in FIG. 150B. This optical sheet 222B hasthe same operation as the optical sheet 222B shown in FIG. 150B and hasthe feature of the transparent material layer 292 explained withreference to FIGS. 149A to 149E.

The optical sheet 222B shown in FIG. 150D is formed by adding atransparent material layer 294 having the scattering property on theprojections 234B formed without the base sheet portion 228 in the sameway as the projections 234 of the optical sheet 222B shown in FIG. 148B.This optical sheet 222B has the same operation as the optical sheet 222Bshown in FIG. 150B.

The optical sheet 222B shown in FIG. 150E is formed by adding a layer294 having the scattering property on the projections 234B formedwithout the scatter material 232. The layer 294 having the scatteringproperty contains the scattering material particles 232A. The scatteringmaterial particles 232A comprises a group of a plurality of smallscattering material particles adhering to one another, in the same wayas the scattering material particles 232 of the projections 234 shown inFIG. 148A.

FIG. 151 shows an example of the optical sheet 222 produced using amesh. In the production of the optical sheet 222, the base sheet portion228 is first formed on the acrylic substrate 238, and the diffusionportion 230 is then formed on the base sheet portion 228 using the mesh40 (mesh No. 300). In this case, the ink 242 is applied under thecondition where the mesh 240 is disposed on the base sheet portion 228,and the mesh 240 is left on the base sheet portion 228 even after theink 242 is dried and cured. The ink is a transparent ink containing anacrylic resin as a base component, and beads having diameters of severalmicrons as the scattering material particles 232. The filaments of themesh 240 do not contain the scattering material particles. The ink 242comprises a material having a refractive index different from therefractive index of the mesh 240. The portion of the ink 242 positionedbetween the filaments of the mesh 240 operates in the same way as theprojection 234 shown in FIG. 119, and the filaments of the mesh 240operate in the same way as the transparent material layer 292 shown inFIG. 149A.

As a modified example of the optical sheet 222 shown in FIG. 151, it isalso possible to employ the construction in which the ink 242 does notcontain the scattering material particles 232 but the filaments of themesh 240 contain the scattering material particles 232. In this case,each filament of the mesh 240 operates as the projection 234, and theportion of the ink 242 positioned between the filaments of the mesh 240operates in the same way as the transparent material layer 292 in FIG.149A.

In the example shown in FIG. 151 and its modified example, the linewidth of the filament of the mesh 240 and the thickness of the mesh 240preferably have the relation, {opening width/(line width+openingwidth)}≧10(%) and the relation, {thickness of mesh 240/(linewidth+opening width)} 40(%).

FIGS. 152A and 152B show another embodiment of the optical sheet 222.This optical sheet 222 includes the diffusion portion 230 having theprojections 234 containing the scattering material particles 232 (seeFIG. 119) and the valley portions 236. FIGS. 152A and 152B show aproduction process of this optical sheet 222. In FIG. 152A, the ink 242Ais applied to the diffusion portion 230 having the projections 234. Theink 242A comprises scattering material particles 242B contained in adispersion base material. The dispersion base material comprises abinder having an adhering action and a solvent. The amount of the binderis considerably small to such an extent as to be capable of fixing thescattering material particles 232A with one another. Since thescattering material particles 242B are heavier than the solvent, theyare distributed along the surfaces of the projections 234 and the valleyportions 236.

In FIG. 152B, the optical sheet 22 is turned upside down so that theapplied ink 242A comes to the lower side of the diffusion portion 230,and the solvent is evaporated. The scattering material particles 242Bpositioned on the valley side 236 move along the surface of the valleyportions 236 and travel to below the projections 234. As the solventevaporates, the surface of the ink 242A comes close to the surface ofthe diffusion portion 230. As a result, the scattering materialparticles 242B are gathered together into a group of scattering materialparticles below the projections 234. In this way, the effect obtained isthe same as when the height of the projections 234 having the scattermaterial 232 increases.

FIGS. 153A to 153C shows another embodiment of the optical sheet. In theoptical sheets 222 and 222A of the foregoing embodiments, theprojections 234 contain the scattering material particles 232, or thelayer 294 having the scattering property is provided onto theprojections 234B. The scattering material particles 232 or 232A make therefractive index of the projections 234 or 234B non-uniform and scatterthe ray of light. Therefore, the projections 234 or 234B can beexpressed as the portions having the non-uniform refractive index.Furthermore, the portions having the non-uniform refractive index do notalways contain the scattering material particles 232 or 232A.

The optical sheet 222B shown in FIG. 153A includes a diffusion portions230 having a plurality of spaced apart or periodically arranged portions234C having non-uniform refractive index, and portions 296 having auniform refractive index and positioned between the portions 234C havingnon-uniform refractive index. The portions 234C having the non-uniformrefractive index can be arranged in the same way as the projections 234or 234B in the previous embodiment. Since the refractive index in theportions 234C is non-uniform, the portions, 234C having the non-uniformrefractive index operate in the same way as the projections 234 or 234Bcontaining the scattering material particles 232 or 232A in theforegoing embodiments. Therefore, the operation of the optical sheet 222shown in FIG. 153A is identical to the operation of the foregoingoptical sheets 222 or 222A. Transparent sheets 298A and 298B arearranged on the optical sheet 222B on the upper and lower sides thereof.

FIG. 153B shows a production example of the optical sheet 222B shown inFIG. 153A. A transparent UV curable resin 296A is sandwiched between thetransparent sheets 298A and 298B, and ultraviolet light is irradiatedusing a mask 300. The UV curable resin 296A is cured by the irradiationof the UV light, but the refractive index becomes non-uniform at theportions 234C to which the UV light is strongly irradiated through theopen portions of the mask 300, and becomes uniform at other portions.

FIG. 153C shows another production example of the optical sheet 222B. Atransparent UV curable resin 296A is sandwiched between transparentsheets 298A and 298B. While a sheet 302 having needle-like projections302A presses the transparent sheet 298B to apply the stress to the UVcurable resin 296A, the UV ray is irradiated from the side of thetransparent sheet 298A. The UV curable resin 296A is cured by theirradiation of the UV ray. In this case, the refractive index becomesnon-uniform at the portions to which the stress is applied and the UVray is irradiated, and becomes uniform at other portions.

FIGS. 154 to 157D show the optical sheet according to the eighthembodiment of the present invention. FIG. 154 is a partial perspectiveview of the optical sheet, and FIG. 155 is a partial enlarged sectionalview of the optical sheet of FIG. 154. The optical sheet 222C includes adiffusion portion 330 having a plurality of spaced apart wall members304 having scattering property, and openings 306 formed between the wallmembers 304. The wall members 304 having the scattering property includescattering material particles 232. Two wall members 304 extendvertically in parallel with each other, and have the same height, andeach opening 306 is formed between two wall members 304 and extendsthrough the optical sheet 222C. In FIG. 154, the diffusion portion 330is formed into a honeycomb structure, and each opening 306 is formed byfour wall members 304 (two sets of opposing wall members 304). Thisoptical sheet 222C can be used suitably while being placed on the lightguide plate 216, for example, though this arrangement is notrestrictive.

Each wall member 304 has first and second side surfaces 304 a and 304 bopposing each other. The wall member 304 is formed in such a fashionthat the ray of light is substantially scatter-reflected by the firstand second side surfaces 304 a and 304 b. In FIG. 155, the ray of lightdd outgoing from the light guide plate 216 outgoes from the opticalsheet 222C within a predetermined angle range a without impingingagainst the wall member 304 in the same way as the ray of light aaexplained with reference to FIG. 119. The ray of light ee outgoing fromthe light guide plate 216 is made incident to the wall member 304 in thesame way as the light bb explained with reference to FIG. 119, and thelight made incident to the wall member 304 is scatter-reflected by thefirst and second side surfaces 304 a and 304 b. A part of the light thusscatter-reflected outgoes from the optical sheet at a relatively smallangle to the normal direction NN, and the rest of the light is madeincident to other wall member 304 at relatively large angles to thenormal direction NN and further scatter-reflected, and outgoing of lightand scatter-reflection are repeated. In this way, the light outgoingfrom the optical sheet 222C is provided with strong directivity in thenormal direction NN.

If the light having relatively large angles to the normal direction NNtransmit across and through the wall member 304, the effect ofdirectivity is low, so the wall member 304 is preferably constructed toprevent the light from substantially transmitting across and through thewall member 304. To this end, the scattering material particles 232 ofthe wall members 304 are arranged in the highest possible density sothat the light is substantially back scattered at the first and secondside surfaces 304 a and 304 b. It is advisable to arrange a memberpermitting no passage of the ray of light or a core member reflectingthe ray of light in the center of the wall member 304, and thescattering material particles 232 are arranged on both sides of thismember.

FIG. 156 shows a modified example of the optical sheet of FIG. 154. Theoptical sheet 222C includes a diffusion portion 230 having a pluralityof spaced apart wall members 304 having scattering property, andopenings 306 formed between the wall members 304. The diffusion portion230 further includes a plurality of beht sheets 310 that are bent intothe shape of a folding screen, and each section extends between thebending positions of each bent sheet 310 serves as each wall member 304.The bent sheets 310 are arranged in parallel with one another, and anopening 306 is formed between the wall members 304 of two bent sheets310.

The operation of this optical sheet 22C is the same as the operation ofthe optical sheet 222C shown in FIG. 154. In the optical sheet 222 cshown in FIG. 156, the gap between two opposing wall members 304 isdifferent between the X direction and the Y direction, and therefore,anisotropy can be imparted to the brightness distribution of theoutgoing ray of light. Incidentally, the gap between the two projections234 can be varied in the X direction and in the Y direction (or in thelongitudinal direction and in the transverse direction) in all of theforegoing embodiments in the same way as in this embodiment. It isobvious from the principle of the present invention that a mazestructure too, provides the same effect.

FIGS. 157A to 157D show another example of the optical sheet and itsproduction method. In FIG. 157A, a scattering sheet 312 containingscattering material particles 232 is prepared, and the scattering sheet312 is shaped into a corrugated sheet 312A using an embossing roller314. In FIG. 157B, the flat scattering sheet 312 and the corrugatedsheet 312A are bonded to obtain a laminate sheet 312B. In FIG. 157C, alarge number of laminate sheets 312B are laminated to obtain a laminatesheet 312C. In FIG. 157D, the laminate sheet 312C is turned 90 degreesfrom the state shown in FIG. 157C, and is cut into a thickness HH in aplane perpendicular to the sheet of the drawing to obtain optical sheets222C. The thickness HH corresponds to the height of the optical sheet222 c as viewed under the condition shown in FIG. 154. When the opticalsheet 222C shown in FIG. 157D is viewed from the direction of arrow KK,it appears as shown in FIG. 157C. The optical sheet 222C has wallmembers 304 and openings 306.

FIG. 158 is a sectional view showing a backlight (illumination device)according to the ninth embodiment of the present invention. In thisdrawing, the backlight 270 includes a light guide plate (optical member)272. FIG. 158 shows an example of the backlight including a thin lightguide plate 272. Even though the light guide plate 272 is thin, thebacklight can input a large quantity of light.

The light guide plate 272 comprises a flat sheet-like body having alight guide region 274 and a light turning region 276. The light guideregion 274 and the light turning region 276 are formed as an opticallycontinuous region. The light guide region 274 is a region substantiallytransparent to the ray of light having a predetermined wavelength, andthe light turning region 276 is a region having a non-uniform refractiveindex. The light turning region 274 has a plurality of spaced apartportions having a non-uniform refractive index and extending along afirst line which is not parallel to the flat sheet-like body, andportions having a uniform refractive index and positioned between theportions having the non-uniform refractive index.

The light guide region 274 is made of a transparent resin such as anacrylic resin in the same way as the conventional light guide plate. Thelight turning region 276 has a construction similar to that of theoptical sheet 222 explained with reference to FIGS. 116 to 153C. Inother words, the light turning region 276 has a base sheet portion 228Xand a diffusion portion 230X. The diffusion portion 230X has projections234X containing scattering material particles 232 (or portions 234Chaving a non-uniform refractive index). The scattering materialparticles 232 are omitted in FIG. 158. The projections 234X are arrangedwith gaps between them along a line parallel to the flat sheet-likebody, and are inclined at a certain angle to that line. Incidentally,the base sheet portion 228X can be omitted in this embodiment in thesame way as the examples of the optical sheet 22 explained above.

The backlight 270 further includes a light source 278 comprising a lamp278A such as a cold-cathode fluorescent tube or a hot-cathodefluorescent tube and a reflector 278B. The light source is disposed onor near the light turning region 276 of the light guide plate 272, andthe reflector 278B substantially encompasses the lamp 278A and the lightturning region 276. The ray of light outgoing from the lamp 278A and theray of light reflected by the reflector 278B are made incident to thelight turning region 276. In the construction including the projections234X containing the scattering material particles 232 in the same way asthe optical sheet 222 of the foregoing embodiments, a part of the lightoutgoing from the valley portion between two projections 234X outgoeswithout contacting the projections 234X, and another part of the lightis made incident to the projections 234X and is scattered. A part of thescattered light outgoing from the projection 234X outgoes withoutcontacting the projections 234X, and another part of the scattered lightis again made incident to the projections 234X and is scattered. In thisway, the ray of light is provided with directivity in the extendingdirection of the projections 234X, and travels from the light turningregion 276 to the light guide region 274. Even when the light guideplate 272 is very thin, the ray of light is efficiently inputted fromthe light source 278 into the light guide plate 272.

FIG. 159 shows a modified example of the backlight 270. This example isdifferent from the example shown in FIG. 158 in that the light source278 comprises an LED device 278C. In FIGS. 158 and 159, the base sheetportion 228X and the diffusion portion 230X are constituted into a unit,and the backlight can be fabricated by bringing this unit into opticallyclose contact with the surface of the light guide plate 272 in the lightturning region 276. Alternatively, the base sheet portion 228X, thediffusion portion 230X, and the transparent resin layer (refer to theresin layers 264 and 265 in FIG. 146) are constituted into a unit andthis unit is brought into optically close contact with the end portionof the light guide region 274 of the light guide plate 272 to form thelight turning region 276.

FIG. 160 shows a modified example of the backlight 270 shown in FIG.158. In this example, the light turning region 276 comprises thediffusion portion 230 having the projections 234X containing thescattering material particles 232. The projections 234X are spaced apartfrom each other along a first line perpendicular to the flat sheet-likebody, and are arranged perpendicularly to the first line. A reflectingmirror 280 is disposed at the end of the light turning region 276 on theside opposite to the light guide region 274. The projections 234X areformed as layers that are relatively elongated and have a large width.The transparent resin layers exist between the adjacent layers. Theoperation of this example is the same as that of the foregoing example.In the example shown in FIG. 160, the light source 278 comprises thelamp 278A. The example shown in FIG. 161 is the same as the exampleshown in FIG. 160, with the exception that the light source 278comprises the LED device 278C.

FIG. 162 shows a still modified example of the backlight 270. Thisexample is the same as the example shown in FIG. 158 with the exceptionthat the base sheet portion 228X is disposed on the side near the lightsource 278. The operation of this example is the same as that of theforegoing example.

FIG. 163 shows a still another modified example of the backlight 270.The light source 278 is disposed by the side of the light guide plate272 or, in other words, by the side of the light turning region 276. Theoutgoing light from the lamp 278A and the light reflected reflector 278Bare made incident to the side surface, the upper surface and the lowersurface of the light turning region 276. The operation of this exampleis the same as that of the foregoing example.

In FIGS. 158 to 163, the light source 278 is disposed on only one endside of the light guide plate 272 but light guides may be disposed onboth end sides of the light guide plate 272. It is further possible toprovide a light absorption member covering the end portions of thereflector 278B put on the flat sheet-like body and to prevent leak ofthe ray of light from the gap between the flat sheet-like body and thereflector 278B.

As explained above, the present invention can acquire an optical sheetand an illumination device which have a suitable brightness distributionsuch that the brightness is higher in the normal direction andprogressively decreases with an increasing angle from the normaldirection, and which can be economically produced. Further, the presentinvention can provide an optical member capable of inputting largequantities of light even when a light guide plate is thin.

FIG. 164 is a perspective view showing the notebook type personalcomputer including the light source device according to the tenthembodiment of the present invention. FIG. 165 is a perspective viewshowing the monitor including the light source device according to thesimilar embodiment of the present invention.

In FIG. 164, the notebook type personal computer 401 comprises a body403 having a keyboard 402 and electronic circuits, and a display part405 having a display device 404 such as a liquid crystal display device.The display part 405 has a light source device 418. The notebook typepersonal computer 401 of FIG. 164 includes one light source device 418,but it is possible to arrange two light source devices 418, as in thecase of the monitor 406 of FIG. 165.

In FIG. 165, the monitor 406 comprises a body 408 having a displaydevice 407 such as a liquid crystal display device and electroniccircuits. The body 408 has light source devices 418. The monitor 406 ofFIG. 165 includes two light source devices 418, but it is possible toarrange one light source device 418, as in the case of the notebook typepersonal computer 401 of FIG. 164.

FIG. 166 is a plan view of the light guide plate and the light sourcedevice of the display device of FIG. 164, and FIG. 167 is a sectionalview of the light guide plate and the light source device of FIG. 166.In FIGS. 166 and 167, the display device 404 includes a liquid crystaldisplay panel 412 and a backlight 414. The backlight 414 includes alight guide plate 416, the light source devices 418 arranged at eitherside of the light guide plate 416, a scattering reflection plate 420arranged below the light guide plate 416, and a scattering plate 422arranged above the light guide plate 416.

The light source device 418 comprises a discharge tube 424 and areflector 426. A part of the ray of light outgoing from the dischargetube 424 is made directly incident to the light guide plate 416, andanother part of the light outgoing from the discharge tube 424 isreflected by the reflector 426 to be made incident to the light guideplate 416. The light travels within the light guide plate 416, isreflected by the scattering reflection plate 420 to outgo from the lightguide plate 416 toward the liquid crystal display panel 412, and is madeincident to the liquid crystal display panel 412 after being scatteredby the scattering plate 422. The liquid crystal display panel 412 formsan image, and the light supplied from the light source device 414illuminates the image formed by the liquid crystal display panel 412, sothat a viewer can see a bright image.

FIG. 168 is a sectional view showing the discharge tube 424, and FIG.169 is a sectional view showing the light source device 414 includingthe discharge tube 424 and the reflector 426. FIG. 170 is a sectionalview of the light source device 414, taken along the line VII-VII inFIG. 169. The discharge tube 424 comprises a cold-cathode tube called afluorescent lamp electrodes 424A made of metal such as Ni or W arearranged on the ends of the discharge tube 424. Lean gas (such as Ar orNe) and mercury 28 are inserted and sealed in the discharge tube 424,and a fluorescent material is coated on the inner surface of thedischarge tube 424. The reflector 426 comprises an aluminum mirror, forexample, and has a cross-sectional shape such a U-shape to cover thedischarge tube 424.

Support members 425 are arranged on the discharge tube 424 near theelectrodes 424A for supporting the discharge tube 424 to the reflector426. The inner surface of the support member 425 is in close contactwith the discharge tube 424, and the outer surface of the support member425 is in close contact with the reflector 426. A portion of theelectrode 424A is within the discharge tube 424, and another portion ofthe electrode 424A extends to the exterior of the discharge tube 424through the end of the discharge tube 424 and the end of the supportmember 425.

The support member 425 is formed of a heat insulating structure so as toprevent a temperature drop of a portion of the discharge tube 424 nearthe electrode 424A. In this embodiment, the support member 425 is madeof a material having high heat insulating property and high withstandingproperty to voltage. For example, the support member 425 is made ofAramid Paper (Nomex sheet of Dupont Corporation). The support member 425can be made of glass wool.

In the structure in which the discharge tube 424 is supported to thereflector 426 by the support members 425, there is a tendency that heatof the discharge tube 424 is thermally conducted to the reflector 426through the support members 425, and further from the reflector 426 tothe housing of the display device, so that the temperature of a portionof the discharge tube 424 near the electrodes 424A may be reduced.

Generally, the operating life of the discharge tube 424 will end if themercury contained in the discharge tube 424 is consumed. Consumption ofmercury occurs in such a manner that gaseous mercury in the dischargetube 424 reacts with particles of metal of the electrodes 424A (forexample, Ni) caused by sputtering with electrons and is adhered to theinner surface of the discharge tube 424. A sufficiently large amount ofmercury is usually inserted in the discharge tube 424, and it takes muchtime for mercury to be consumed, and therefore, the operating life ofthe discharge tube 424 is guaranteed to some extent. However, there areseveral discharge tubes 424 among many discharge tubes 424 which mayhave had in an extremely shortened operating life. The object of thisembodiment is to prevent the discharge tubes 424 from ending in ashortened operating life.

According to the inventor's study, a shortened operating life of thedischarge tubes 424 can be caused in the following way. Conventionalsupport members are made of silicone so as to withstand high voltageapplied to the electrodes 424A. The temperature at the portion of thedischarge tube 424 near the electrodes 424A is fundamentally higher thanthat at the other portion of the discharge tube 424 since much heat isgenerated at that portion, but in the structure in which the dischargetube 424 is supported to the reflector 426 by the silicone supportmembers, heat of the discharge tube 424 is thermally conducted very muchto the reflector 426 through the silicone support members since siliconehas good heat conductivity, and the temperature of the portion of thedischarge tube 424 near the electrodes 424A may be reduced to the lowestvalue. Therefore, liquid mercury is collected at the portion of thedischarge tube 424 near the electrodes 424A where the temperature islowest. If particles of metal of the electrodes 424A caused bysputtering are deposited on liquid mercury collected at the portion ofthe discharge tube 424 near the electrodes 424A, and produce a thinmembrane on liquid mercury. This membrane prevents mercury fromevaporating and the amount of gaseous mercury is decreased. Thedischarge tube 424 becomes dark in the activated condition if the amountof gaseous mercury is decreased, and the operating life thereof isshortened.

In this embodiment of the present invention, the support member 425 isformed of a heat insulating structure, so heat of the discharge tube 424is thermally conducted less to the reflector 426 through the supportmembers, and the temperature of the portion of the discharge tube 424near the electrodes 4244 is not reduced to the lowest value. Thetemperature at the portion of the discharge tube 424 near the electrodes424A is fundamentally higher than that at the other portion of thedischarge tube 424 since much heat is generated at that portion, and theposition at which the temperature becomes lowest in the discharge tube424 is located to positioned shifted inwardly with respect to the regionin which the support member 425 extends. Therefore, liquid mercury isnot collected at the portion of the discharge tube 424 near theelectrodes 424A.

On the other hand, metal of the electrodes 424A is sputtered withelectrons during discharge and particles of metal of the electrodes 424Aare deposited on the inner surface of the discharge tube 424. The regionin which particles of metal of the electrodes 424A are deposited on theinner surface of the discharge tube 424 is restricted to the regionwithin a restricted distance from the end of the electrode 424A. Forexample, in the case of the discharge tube 424 having the diameter of 5mm, the region in which particles of metal of the electrodes 424A aredeposited on the inner surface of the discharge tube 424 is the regionwithin 10 mm from the end of the discharge tube 424 or within 5 mm fromthe end of the electrode 424A.

Liquid mercury is not collected in the region in which particles ofmetal of the electrodes 424A are deposited, so liquid mercury is notenclosed by particles of metal. Therefore, according to the presentinvention, most liquid mercury can continue to evaporate and the amountof gaseous mercury is not reduced, so the operating life of thedischarge tube 424 is not shortened.

FIG. 171 is a sectional view of the light source device 418 includingthe discharge tube 424 and the reflector 426 according to a modifiedembodiment, and FIG. 172 is a sectional view of the support member ofFIG. 171. Support members 425 are arranged on the discharge tube 424near the electrodes 424A for supporting the discharge tube 424 to thereflector 426. The support member 425 is formed of a heat insulatingstructure so as to prevent a temperature drop of a portion of thedischarge tube 424 near the electrode 424A. In this embodiment, thesupport member 425 is made of silicone, similar to a conventionalsupport member, but the support member 425 has a hollow portion 425B torealize the heat insulating structure. The operation of this embodimentis similar to that of the previous embodiment.

FIG. 173 is a sectional view of the light source device 418 includingthe discharge tube 424 and the reflector 426 (reflector 426 is omittedin FIG. 173) according to a modified embodiment. In this embodiment, thedischarge tube 424 is partially formed of a heat insulating structure soas to prevent a temperature drop of a portion of the discharge tube 424near the electrode 424A of the discharge tube 424. In other words, theend portion of the discharge tube 424 is formed in a double-tubestructure having an outer tube portion 424 o and an inner tube portion424 i, so that a heat insulation part is provided between the outer tubeportion 424 o and the inner tube portion 424 i. The support member 425is arranged around the outer tube portion 424 o and supports thedischarge tube 424 to the reflector 426. The operation of thisembodiment is similar to that of the previous embodiment.

FIG. 174 is a sectional view of the light source device 418 includingthe discharge tube 424 and the reflector 426 according to a modifiedembodiment. In this embodiment, the support members 425 are arranged atinward positions from ends of electrodes 424A of the discharge tube 424so as to prevent a temperature drop of a portion of the discharge tube424 near the electrode 424A of the discharge tube 424. As describedabove, the region in which particles of metal of the electrodes 424A aredeposited on the inner surface of the discharge tube 424 is restrictedto the region within a restricted distance from the end of the electrode424A. The support members 425 are arranged on the outside of the regionin which particles of metal of the electrodes 424A are deposited (i.e.,inward positions).

In this case, the support members 425 are not necessarily made of a heatinsulating material, and are made of silicone. Therefore, thetemperature is lowest at the portion of the discharge tube 424 where thesupport members 425 are located due to the thermal conduction throughthe support members 425, as described above. However, the portion of thedischarge tube 424 where the temperature is out of the region in whichparticles of metal of the electrodes 424A are deposited on the innersurface of the discharge tube 424, so liquid mercury is not enclosed byparticles of metal. Therefore, according to the present invention, mostliquid mercury can continue to evaporate and the amount of gaseousmercury is not reduced, so the operating life of the discharge tube 424is not shortened.

FIG. 175 is a sectional view of the light source device 418 includingthe discharge tube 424 and the reflector 426 according to a modifiedembodiment. In this embodiment, the light source device 418 comprisessupport members 425 arranged at positions near electrodes 424A of thedischarge tube 424 for supporting the discharge tube 424 to thereflector 426, and a heat conduction member 432 contacting a centralportion of the discharge tube 424. The support members 425 are made ofsilicone. The heat conduction member 432 is made of silicone having goodheat radiating property. Alternatively, it is possible to arrange finson the heat conduction member 432 or to cool the heat conduction member483 by cooling air blown by a fan.

The heat conduction member 432 also contacts the reflector 426 andreleases heat of the central portion of the discharge tube 424 to thereflector 426, so that the temperature at the central portion of thedischarge tube 424 is lowest. Therefore, liquid mercury is not collecteda portion of the discharge tube 424 near the electrodes 424A, and liquidmercury is not enclosed by particles of metal. Therefore, according tothe present invention, most liquid mercury can continue to evaporate andthe amount of gaseous mercury is not reduced, so the operating life ofthe discharge tube 424 is not shortened.

Also, by producing a lowest temperature point at the central portion ofthe discharge tube 424, mercury evaporates mainly at the lowertemperature portion and resultant gaseous mercury diffuses in the wholedischarge tube 424. The diffused gaseous mercury also returns to thelower temperature portion. In this way, gaseous mercury is uniformlydistributed in the whole discharge tube 424, and the temperature and thepressure of gaseous mercury are constant in the whole discharge tube424. That is, it is possible to control the temperature of gaseousmercury, by producing lower temperature point. The brightness of the rayof light outgoing from the discharge tube 424 becomes maximum at theoptimum concentration of gaseous mercury and the correspondingtemperature in the discharge tube 424, and the brightness of the ray oflight outgoing from the discharge tube 424 is lower than the maximumvalue if the concentration of gaseous mercury is higher or lower thanthe optimum concentration or the temperature in the discharge tube 424is higher or lower than the maximum value. In the this embodiment, it ispossible to acquire the maximum brightness of the light outgoing fromthe discharge tube 424, by by producing a lower temperature point in thedischarge tube 424 to thereby setting the temperature in the dischargetube 424 at or near the optimum value.

As explained, according to these embodiment, it is possible to obtain alight source device having a discharge tube having a long operatinglife.

1. A backlight comprising: a discharge tube containing mercury in whichalmost all liquid mercury except for an amount of gaseous mercury at atime of discharge is collected at a first position apart from an end ofsaid discharge tube; a cooling device for cooling said first position ofsaid discharge tube; a reflector for reflecting a ray of light radiatedfrom said discharge tube and a light guide plate receiving the ray oflight radiated from said discharge tube and the ray of light reflectedby said reflector; and a width of the cooling device in a lengthdirection of the discharge tube is not more than 7.5 times an innerdiameter of the discharge tube.
 2. A backlight according to claim 1,wherein said cooling device comprises a heat conduction memberpositioned to come into contact with said first position of saiddischarge tube, or positioned in the proximity of said first position ofsaid discharge tube.
 3. A backlight as set forth in claim 1, wherein awidth of the cooling device in a length direction of the discharge tubeis not less than an inner diameter of the discharge tube.
 4. A backlightcomprising: a discharge tube containing mercury in which almost allliquid mercury except for an amount of gaseous mercury at a time ofdischarge is collected at a first position apart from an end of saiddischarge tube; a cooling device for cooling said first position of saiddischarge tube; and a reflector for reflecting a ray of light radiatedfrom said discharge tube and a light guide plate receiving the ray oflight radiated from said discharge tube and the ray of light reflectedby said reflector, wherein said discharge tube has electrodes atopposite ends thereof, and said first position is located within a rangespaced apart by 10D or by at least 0.25L from a tip of said electrode ateach end of said discharge tube, where an inner diameter of saiddischarge tube is D and a distance between the electrodes at oppositeends of said discharge tube is L.
 5. A backlight according to claim 4,wherein said first position possesses a limited portion in said range ora whole portion in said range.
 6. A backlight comprising: a dischargetube containing mercury in which almost all liquid mercury except for anamount of gaseous mercury at a time of discharge is collected at a firstposition apart from an end of said discharge tube; a cooling device forcooling said first position of said discharge tube; and a reflector forreflecting a ray of light radiated from said discharge tube and a lightguide plate receiving the ray of light radiated from said discharge tubeand the ray of light reflected by said reflector, wherein said mercurycomprises mercury particles having a size of not greater than 0.2 mm, orsaid mercury soaks into a fluorescent material applied to an inner wallof said discharge tube.
 7. A backlight comprising: a discharge tubecontaining mercury in which almost all liquid mercury except for anamount of gaseous mercury at a time of discharge is collected at a firstposition apart from an end of said discharge tube; a cooling device forcooling said first position of said discharge tube; and a reflector forreflecting a ray of light radiated from said discharge tube and a lightguide plate receiving the ray of light radiated from said discharge tubeand the ray of light reflected by said reflector, wherein said dischargetube contains a rare gas, and said rare gas does not contain argon.
 8. Abacklight according to claim 7, wherein electrodes of said dischargetube further comprise a carbon nanotube.
 9. A backlight comprising: adischarge tube containing mercury in which almost all liquid mercuryexcept for an amount of gaseous mercury at a time of discharge iscollected at a first position apart from an end of said discharge tube;and a cooling device for cooling said first position of said dischargetube, wherein said discharge tube contains a rare gas, and said rare gasdoes not contain argon, and wherein said cooling device comprises athermo-chromic material coming into contact with said discharge tube, ora transparent material containing a thermo-chromic material.
 10. Abacklight comprising: a discharge tube containing mercury in whichalmost all liquid mercury except for an amount of gaseous mercury at atime of discharge is collected at a first position apart from an end ofsaid discharge tube; a cooling device for cooling said first position ofsaid discharge tube; and a reflector for reflecting a ray of lightradiated from said discharge tube and a light guide plate receiving theray of light radiated from said discharge tube and the ray of lightreflected by said reflector, wherein said cooling device includes acooling capacity varying mechanism.
 11. A backlight comprising: adischarge tube containing mercury in which almost all liquid mercuryexcept for an amount of gaseous mercury at a time of discharge iscollected at a first position apart from an end of said discharge tube;and a cooling device for cooling said first position of said dischargetube, wherein said cooling device includes a movable heat conductionmember.